docs: expand EXTERN_C guard macros in the Doxygen API build

[DavidCozens]

What

Doxygen runs (via mkdoxy) with MACRO_EXPANSION off, so the unexpanded EXTERN_C_BEGIN token gets glued onto the first declaration in each public header. For example, SolidSyslogSsize in SolidSyslogStream.h rendered with type EXTERN_C_BEGIN typedef intptr_t instead of intptr_t. This hit ~37 headers — whichever symbol immediately follows EXTERN_C_BEGIN in each (SolidSyslogCrc16_Compute, SolidSyslogFatFsFile_Create, SolidSyslogConfigLockFunction, …).

Fix

In mkdocs.yml under doxy-cfg:

• MACRO_EXPANSION: true — let the preprocessor expand macros
• EXPAND_ONLY_PREDEF: true — but only the ones we list, so the SOLIDSYSLOG_* constants stay documented by name rather than expanded to their values
• PREDEFINED: "EXTERN_C_BEGIN= EXTERN_C_END=" — expand both C-linkage guard macros to nothing

Doxygen then sees plain file-scope C declarations, which matches OPTIMIZE_OUTPUT_FOR_C.

Verification

Built the docs locally in the mkdocs-mkdoxy:sha-34173c0 container with mkdocs build --strict:

• mkdocs build --strict passes
• Regenerated XML: <definition>EXTERN_C_BEGIN count dropped from 37 → 0; SolidSyslogSsize type is now typedef intptr_t
• Rendered HTML for the Stream page shows a clean SolidSyslogSsize with zero EXTERN_C_BEGIN leakage

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Improved API reference generation for C declarations wrapped in linkage guards.
  • Public type definitions are now displayed more clearly without carrying over internal linkage macros.
  • Documented constants continue to appear by their defined names rather than expanded values.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: dd796e53-d4a4-46d4-bcd0-9ead8df4c227

📥 Commits

Reviewing files that changed from the base of the PR and between ed5ffeb and bc0fe73.

📒 Files selected for processing (1)

• mkdocs.yml

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Walkthrough

The Doxygen configuration now enables controlled macro expansion and explicitly removes EXTERN_C_BEGIN and EXTERN_C_END during parsing, allowing wrapped C declarations to render without unexpanded guard tokens.

Changes

Doxygen documentation configuration

Layer / File(s)	Summary
Configure C-linkage macro expansion mkdocs.yml	Enables controlled macro expansion and defines EXTERN_C_BEGIN and EXTERN_C_END as empty values for Doxygen parsing, while retaining named documentation for other macros.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, Conventional Commits-formatted, and accurately describes the Doxygen macro-expansion fix.
Description check	✅ Passed	The description covers purpose, change, verification, and affected area in substance, though it uses non-template headings.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/doxygen-externc-macro-expansion

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[github-actions]

☀️   Quality Summary

   🚦   build-linux-gcc: 100% successful (✔️ 1522 passed)
   🚦   build-freertos-host-tdd-plustcp: 100% successful (✔️ 1873 passed)
   🚦   build-linux-clang: 100% successful (✔️ 1453 passed)
   🚦   sanitize-linux-gcc: 100% successful (✔️ 1453 passed)
   🚦   integration-linux-openssl: 100% successful (✔️ 16 passed)
   🚦   integration-linux-mbedtls: 100% successful (✔️ 14 passed)
   🚦   integration-windows-openssl: 100% successful (✔️ 16 passed)
   🚦   bdd-linux-syslog-ng: 94% successful (✔️ 49 passed, 🙈 3 skipped)
   🚦   bdd-windows-otel: 88% successful (✔️ 46 passed, 🙈 6 skipped)
   🚦   bdd-freertos-qemu-plustcp: 87% successful (✔️ 45 passed, 🙈 7 skipped)
   🚦   bdd-freertos-qemu-lwip: 87% successful (✔️ 45 passed, 🙈 7 skipped)
   🚦   build-windows-msvc: 100% successful (✔️ 1298 passed)
   🚦   build-linux-tunable-override: 100% successful (✔️ 1453 passed)
   ⚠️   Clang-Tidy: No warnings
   ⚠️   CPPCheck: No warnings

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