Keep image content theme parse-only

[leehack]

Summary

• stop serializing legacy ImageContent.theme on normal image content blocks
• keep parsing legacy theme values so old icon-shaped payloads remain readable
• leave spec-compliant McpIcon.theme behavior unchanged for advertised icons

Spec context

Both current MCP 2025-11-25 and the draft schema define ImageContent with _meta, annotations, data, mimeType, and type; neither schema includes theme on image content blocks. The theme field belongs on Icon/McpIcon metadata, so this makes the content serializer spec-shaped without breaking legacy parsing.

Part of #177.

Tests

• dart format lib/src/types/content.dart test/types_test.dart
• dart test test/types_test.dart test/mcp_2025_11_25_test.dart test/mcp_2026_07_28_test.dart
• git diff --check
• dart analyze
• dart test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.48%. Comparing base (9bcb972) to head (6357227).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           spec/mrtr-sampling-request-shape     #252      +/-   ##
====================================================================
- Coverage                             88.48%   88.48%   -0.01%     
====================================================================
  Files                                    47       47              
  Lines                                  9354     9353       -1     
====================================================================
- Hits                                   8277     8276       -1     
  Misses                                 1077     1077              

Flag	Coverage Δ
unittests	88.48% <ø> (-0.01%)	⬇️

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[leehack]

Closing this draft micro PR because the RC spec work has been consolidated into a smaller reviewable stack. This change is preserved in #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.