feat: update X-Client-Info to use structured semicolon-delimited metadata

[grdsdev]

Summary

• Consolidates platform and runtime metadata into the X-Client-Info header using semicolon-delimited key=value pairs
• Removes the four standalone X-Supabase-Client-* headers in favor of inline structured metadata
• Applies consistently across all sub-clients: auth, postgrest, storage, functions, and the main client

New format:

X-Client-Info: supabase-py/<component> v<version>; platform=<OS>; platform-version=<release>; runtime=python; runtime-version=<python-version>

Why: Adding new headers is a breaking change for supabase-js in Edge Functions because users must explicitly allow headers in CORS config. Since X-Client-Info is already allowed everywhere, embedding metadata there avoids that problem — matching the approach used in supabase-swift.

Changes

• auth (_async, _sync): Replaced 5-header dict with structured X-Client-Info
• postgrest (_async, _sync): Same
• storage (_async, _sync): Same
• functions (_async, _sync): Same
• supabase/lib/client_options.py: Added platform/runtime metadata to default header
• Tests: Added structured-format and no-separate-headers tests; updated existing tests that asserted the old plain format

Testing

New tests added (all passing)

• TestXClientInfo::test_structured_metadata_format — verifies regex pattern match
• TestXClientInfo::test_no_separate_platform_headers — verifies removed headers are absent
• Functions, storage, postgrest (async + sync) each have equivalent coverage

Test results

• postgrest: 182 passed
• functions: 67 passed, 1 skipped
• storage: 39 passed (integration tests require running server, unchanged)
• supabase: 33 passed, 12 xfailed, 86 xpassed

Acceptance Criteria

• X-Client-Info contains platform, platform-version, runtime, runtime-version as semicolon-delimited key=value pairs
• Separate X-Supabase-Client-* headers are removed
• Applied to all sub-clients (auth, postgrest, storage, functions, main)
• All unit tests pass

Linear Issue

Closes: SDK-904

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