fix: reject unprovable required evidence media types

[Brad-Edwards]

Summary

Reject required evidence media types that cannot be represented by a registered output contract and verified from captured bytes.

Requirement UIDs

• (none — bug/refactor/maintenance run; see Traceability section below)

Related Issues

Closes #1401

ADR Impact

• ADR-021

Changes

• Validate SDL and experiment capture-spec media declarations against the governed output registry, including explicit contract compatibility.
• Publish schema invariant metadata and update the schema publication ledger for both contract schemas.
• Add regression coverage for rejected text, NDJSON, and PCAP declarations and content-backed JSON and JSON Lines proof; correct related fixtures and guidance.

Test Plan

• Unit tests pass
• Integration tests pass if applicable
• Full completion suite required in CI before merge
• No coverage regression

Targeted pytest: 73 passed before review, then 12 passed after the fixture correction. Ruff lint and format, generated-schema drift, schema publication, and the pre-publish hook passed.

Ground Control Checks

• Repository policy checks required in CI before merge
• Pre-push Codex review completed; all findings fixed or dispositioned

Traceability

• IMPLEMENTS: DSL-124 ← implementations/python/packages/raes/evidence_requirements.py, EXP-707 ← implementations/python/packages/raes_contracts/contracts/experiment_capture.py, EXP-708 ← implementations/python/packages/raes_contracts/evidence_output_validation.py
• TESTS: DSL-124, EXP-707, EXP-708, EXP-715 ← implementations/python/tests/test_issue_1401_media_type_coherence.py

Checklist

• Code follows the project's coding standards
• Changelog: owned by Release Please (generated from the Conventional Commit PR title; no per-PR fragment)
• Architectural docs updated if stack, package structure, or key behaviors changed

Documentation

Updated: see diff.