feat(codegen): propagate JSON Schema deprecated:true as // Deprecated: doc comment

[bokelley]

Refs #134

Modifies adcp/schemas/generate.py so that JSON Schema properties with deprecated: true produce a preceding standalone // Deprecated: <description> comment on the generated Go struct field. The preceding-line form is required by Go's documentation comment convention (go.dev/doc/comment §Deprecated) for gopls, go doc, and IDE hover to recognise the deprecation — a trailing inline comment (the existing approach for description text) is not recognised.

No changes to types_gen.go: the current 3.0 bundle has no deprecated: true properties. The generated output will update automatically when the AdCP 3.1 schema bundle lands (download.sh + generate.py re-run, see adcontextprotocol/adcp#4904).

What changed

adcp/schemas/generate.py (8 lines changed in schema_to_struct):

• When prop.get('deprecated', False) is true, emit // Deprecated: <description> on a line preceding the field declaration, and suppress the duplicate trailing inline description comment.
• Strip a redundant leading "Deprecated: " prefix (case-insensitive) from description before using it as the message, to prevent // Deprecated: Deprecated: ... if a schema author sets both deprecated: true and starts the description with "Deprecated: " (the existing axe_include_segment pattern in the 3.0 bundle, lines 2004–2005 of types_gen.go, illustrates this authoring pattern).
• Fallback message when no description is present: "No replacement specified.".

adcp/schemas/test_generate.py (new, 156 lines): Python unittest for schema_to_struct. 7 test cases: basic deprecated field, fallback message, non-deprecated unchanged, no-description, mixed struct regression, double-prefix strip, case-insensitive strip.

adcp/generator_test.go (new, 26 lines): Go test that shells out to python3 -m unittest discover so the Python tests run when cd adcp && go test ./... is executed. Python is already a build-time dependency of this module (CI invokes generate.py and lint.py).

What was tested

• python3 -m unittest discover -s adcp/schemas -p 'test_*.py' -v → 7/7 pass
• cd adcp && go test ./... → all pass (including TestGeneratorDeprecatedPropagation)
• go build ./... (root module) → clean
• go vet ./... (root module) → clean
• cd adcp/schemas && python3 lint.py → no drift (no schema in 3.0 bundle has deprecated: true, so types_gen.go is unchanged)

⚠️ Pre-existing CI gap (adcp/ module, not introduced by this PR): The adcp/ sub-module (adcp/go.mod) has no dedicated CI step in .github/workflows/ci.yml. All 6+ existing test files in adcp/ (e.g. testcontroller_test.go, addtool_test.go, seller_test.go) run only via cd adcp && go test ./... locally. A follow-up CI step analogous to the existing Test context agent / Test bench module steps is needed:

- name: Test adcp module
  working-directory: adcp
  run: go test -race -count=1 ./...

Pre-PR review

• code-reviewer: approved — re.sub double-prefix fix is correct; CI gap is pre-existing infrastructure debt for all adcp/ tests, not a logic issue; nit (doc comment clarity) addressed
• ad-tech-protocol-expert: approved — double-prefix blocker resolved; JSON Schema deprecated → Go // Deprecated: preceding-line mapping is correct per go.dev/doc/comment; back-compat clean on 3.0 bundle
• security-reviewer: approved — no TEE/pinhole surface; safe_comment() sanitization already covers the new deprecated_msg path; no new injection surface

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See adcp#3121
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Session: https://claude.ai/code/session_01TXU3btrUh956CDjk3rm8sL

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