[codex] Add OpenAI compatibility guardrail report

[mimeding]

Business rationale

OpenAI-compatible clients are part of the Osaurus multi-provider harness, and users notice immediately when familiar Chat Completions response shapes, streaming frames, tool-call deltas, or validation errors drift. This PR gives maintainers a repeatable local report with saved artifacts, so API compatibility changes can be reviewed against concrete evidence instead of manual curl snippets.

Coding rationale

The guardrail stays as a script because it needs to exercise a running Osaurus server as a black-box API contract, not import app internals. The Makefile target gives contributors a stable entry point while still allowing HOST, MODEL, OUT_DIR, and REQUIRE_TOOL_CALLS overrides. The script records request, status, headers, body, and streaming chunk artifacts so failures are inspectable after the run. The branch was rebuilt as one clean commit on current origin/main, dropping unrelated stale stack history from earlier PR dependencies.

What changed

• Expanded scripts/openai_compat_report.sh beyond streaming-only checks.
• Added non-streaming Chat Completions response-shape validation.
• Added SSE framing and streaming chunk-shape validation.
• Added OpenAI-style streaming tool-call delta validation with optional REQUIRE_TOOL_CALLS=1.
• Added unsupported request validation/error-shape coverage for n > 1.
• Wrote detailed artifacts under results/openai_compat/ and a Markdown summary at results/openai_compat_report.md by default.
• Added make compat-openai and documented it in docs/DEVELOPER_TOOLS.md.

Validation

• bash -n scripts/openai_compat_report.sh
• shellcheck scripts/openai_compat_report.sh
• make -n compat-openai HOST=http://localhost:1337 MODEL=foundation
• git diff --check HEAD~1..HEAD
• swift build --package-path Packages/OsaurusCore
• swift build --package-path Packages/OsaurusCore -c release
• swift test --package-path Packages/OsaurusCore — initial run hit one unrelated suite issue; immediate rerun passed with 1436 tests in 192 suites.
• Swift-format/swiftlint touched-file checks are not applicable: this PR touches no Swift files.
• CLI tests skipped: this PR touches no CLI package files.

Non-scope

• Does not add a GitHub Actions job for live API compatibility because it requires a running Osaurus server and model.
• Does not change server request/response behavior.
• Does not add generated report artifacts to the repository.

Residual risks

• Tool-call validation can only pass when the selected model actually emits tool calls; by default the script records that as a warning unless REQUIRE_TOOL_CALLS=1 is set.
• The report is a local maintainer guardrail, so future CI automation would need a separate controlled server fixture.

[mimeding]

Author-side cleanup complete; this PR is now ready for review.

Verification:

• GitHub checks are green: test-core, test-cli, swiftlint, shellcheck, update_release_draft, and pr-clean-gate.
• scripts/ci/check-pr-clean.sh osaurus-ai/osaurus 987 reports clean.

Historical failures were from stale DerivedData module resolution and stale FluidAudio locks; current head has the PR cold-DerivedData path and FluidAudio 0.14.3 lock alignment.