Feature/renderers unit tests

[Lppetrov0]

Description

Adds direct unit tests for both renderers, plus rendering robustness fixes for bugs the tests uncovered.

Tests

• New renderer/html_renderer/tests/render_test.c (14 tests) and renderer/md_renderer/tests/render_test.c (13 tests). Each test builds a small DxModel fixture in code and asserts directly on the rendered output — no check.sh or golden-file diff step needed, so those were removed from the component Makefiles.
• Coverage includes: nested directory trees, full vs. minimal symbols, default title fallback, HTML escaping, malformed/self-referencing parent indices, mermaid diagram and cross-reference toggling, sibling ordering, multiple symbols per file, unnamed-field fallbacks, and write-failure behavior.
• Minimal self-contained harness: CHECK* macros use setjmp/longjmp instead of assert(), so a failed check aborts only the current test with a file:line message. All tests still run, every failure is reported, and the binary exits non-zero at the end.
• Removed tests/ from both renderers' .gitignore so the test sources are tracked.

Renderer fixes (found while writing the tests)

• HTML renderer: renders into index.html.tmp and renames it over index.html only on success, so a failed render never leaves a partial page behind or clobbers a previous good one. Write/close errors are now detected and reported to stderr with errno details.
• Markdown renderer: on any write failure, cleans up everything written so far (partial module files, index.md, and now-empty directories) so the caller gets a clean error instead of a partial render. ferror/fclose results are now checked, with stderr diagnostics for unopenable files and over-long output paths.

Build

• Top-level make test now keeps going when one component's tests fail, so every component gets tested, then exits non-zero if any failed (previously it stopped at the first failing component).
• Ignore .vscode/ and remove the tracked c_cpp_properties.json.

Related issues

Closes #

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Documentation
• Refactor / chore (no functional change)

Checklist

• My branch is up to date with main
• I followed the branch naming convention (feature/*, fix/*, docs/*, chore/*)
• Self-reviewed my own code
• Added/updated tests where it makes sense
• Updated documentation where needed
• No secrets, credentials, or proprietary source are committed