fix(contracts): report unquoted invalid dates in experiment specs as invalid yaml

[doublewhy]

Summary

parse_experiment_spec and load_experiment_spec let a bare ValueError escape for an unquoted invalid date. The experiment loader is a PyYAML SafeLoader (YAML 1.1), so description: 2026-02-30 resolves to !!timestamp and construct_yaml_timestamp raises ValueError: day is out of range for month unhandled; likewise for 2026-13-01, 0000-01-01, an out-of-range hour or offset, and an invalid date as a nested mapping key. The MCP experiment_validate tool, which catches only ExperimentSpecValidationError, raised a traceback too.

The loader now maps a ValueError raised while reading the YAML to the existing invalid-yaml ExperimentSpecValidationError, with every YAML failure's fixed message ("Experiment spec YAML is invalid."). The rejected value and Python's message never appear in details; the original error stays on __cause__. Valid unquoted dates are accepted and typed exactly as before.

Requirement UIDs

• None. Loader error-mapping bug fix; Experiment spec loader raises a bare ValueError on an unquoted invalid date #1563 has no ## Requirements section and no requirement scope file.

Related Issues

Closes #1563

ADR Impact

• No ADR required; no ADR text changes. The fix stays inside ADR-074's thin Pydantic loader (section 4), does not echo rejected values (issue-274 preflight, item 2), and adds no exception, diagnostic type or code (issue-164 and issue-652 preflights). The YAML 1.2 Core profile (issue-721 preflight) is SDL-only.

Sources read: YAML 1.1 timestamp type (https://yaml.org/type/timestamp.html; its regexp accepts impossible dates), YAML 1.2.2 section 10.3 (https://yaml.org/spec/1.2.2/; no timestamp tag in the Core schema, so SDL is unaffected), PyYAML 6.0.3 constructor.py (pinned).

Changes

• raes_contracts/experiment_spec.py:parse_experiment_spec: the yaml.YAMLError arm around _load_yaml becomes except (yaml.YAMLError, ValueError) as exc: (same code, message and path, chained from exc). Neither PyYAML error subclasses ValueError, so no existing mapping changes; load_experiment_spec delegates and gets its "<file>: " prefix. Design deviations, recorded in the design note: one widened arm instead of two identical arms; content-addressed captures (since feat(contracts): publish inject readback, correlation and backend declaration contracts #1449) instead of numbered republishing.
• Research captures, collected on a clean checkout of this branch (dev 348c490 plus the fix). The earlier pair, copied from dev's capture, is dropped. Specification coverage: all 6 pinned artifacts ran through tools.specification_coverage._artifacts._execute_artifact (42 stage results: 38 passed, 4 preregistered non-execution), then recompute_analysis (10 of 10 load-bearing passed). Formal validation: the 10 retained SDL cases replayed with replay_case, the 4 production cases ran with their raes processor satisfiability / exploit-path argv, and the protocol's 14 participant test functions ran with pytest (18 passed); 20 of 20 observations equal their expected outcome. Recorded with python -m tools.research_capture record against dev's newest manifests: specification capture-961ac2d4... (template capture-c9ca186a...) and formal capture-68f6095a... (template capture-3bad3945...), each with its capture-records receipt. Drift from the templates: the implementation-surface digest (this fix), the formal retained-case-replay argv and versions.python (3.12.3). No outcome changed.

Done-when matrix (tests in test_issue_1563_experiment_spec_invalid_date.py, CI lane canonical / test-shard):

Done-when (verbatim)	Change	Test; result without the fix	Evidence
parse_experiment_spec and load_experiment_spec raise ExperimentSpecValidationError for an unquoted invalid date such as 2026-02-30.	experiment_spec.py:parse_experiment_spec	test_parse_rejects_unquoted_invalid_timestamp_as_invalid_yaml (6 cases), test_load_rejects_unquoted_invalid_date_with_path, test_mcp_validate_reports_invalid_date_as_validation_error: all 8 fail with a bare ValueError	9 passed locally; CI run 38482420601
A regression test fails without the change and passes with it.	same	the 8 above; the control test_valid_unquoted_date_still_reaches_contract_validation passes either way	stash run
tools/check_specification_coverage.py and tools/check_formal_semantic_validation.py pass.	the two captures	both tools	exit 0 at the head and on the git merge-tree --write-tree origin/dev HEAD tree

Test Plan

• Unit tests pass
• Integration tests pass if applicable
• Completion command passes (canonical / gate passed)
• No coverage regression (canonical / coverage-reduce passed)

uv run --project implementations/python --frozen --all-extras python -m pytest implementations/python/tests/test_issue_1563_experiment_spec_invalid_date.py -q -p no:cacheprovider: 9 passed. Parse cases assert code invalid-yaml and a ValueError cause, the load case path, the MCP case VALIDATION ERROR; all assert the fixed details and no rejected value. The control (2026-02-28) still fails with contract-invalid.

Fail-without-change: with git stash push implementations/python/packages/raes_contracts/experiment_spec.py (restored afterwards), 8 fail and the control passes. Every failure is a bare ValueError from the datetime or timedelta constructors (for example day is out of range for month, year 0 is out of range).

test_experiment_authoring.py, test_example_schema_conformance.py and the new module: 86 passed (no integration-marked tests). tools/check_example_library.py passed. The five evidence modules (test_formal_semantic_validation*.py, test_specification_coverage*.py, test_research_capture.py): 173 passed, and 20 with -m integration. nox -s contracts and nox -s verify-fast-feedback (both -- --base-rev origin/dev), nox -s lint, make policy and ruff (from implementations/python) passed.

CI (checked for this edit): head e4f577b on dev 348c490; run 38482420601 passed every job after one rerun of generic-tool-local-inputs, whose Isabelle mirror download failed its exact payload verification; the same commit passed on rerun; deploy skipped.

Ground Control Checks

• Repository policy command passes

The fail-without-change run is the test-quality evidence; no Ground Control pre-push review was used for this lane.

Traceability

• IMPLEMENTS: implementations/python/packages/raes_contracts/experiment_spec.py
• TESTS: implementations/python/tests/test_issue_1563_experiment_spec_invalid_date.py

Checklist

• Code follows the project coding standards
• FM0: parser-level error mapping; parser unit tests are the required artifact. No executable or runtime adoption is claimed.
• No model or schema changed.
• PR title is a Conventional Commit (fix, patch release).

Documentation

Verified unchanged: the error's docstring already covers a specification that "failed to parse or validate"; no doc describes the loader's exception types.