chore: align tracked pydantic-ai version to 1.104.0 and update parity notes

[Copilot]

This PR ports the v1.87.0 → v1.104.0 update cycle at the repo-tracking/documentation layer: it advances the tracked upstream version and records applicability of the release changes in Vibes parity docs.

• Version tracking

  • Updated .github/pydantic-ai-version.txt from 1.87.0 to 1.104.0.

• Feature parity updates (packages/sdk/docs/reference/features.mdx)

  • Added Claude Opus 4.8 support as ported via provider model ID passthrough.
  • Added v1.104.0 bug-fix entries for:
    • Hybrid OpenRouter/xAI/Bedrock thinking=False forwarding
    • Bedrock single-tool tool_choice cache preservation
  • Marked both bug-fix items as not applicable to Vibes because route-level provider mediation/caching logic is not implemented in this framework layer.

.github/pydantic-ai-version.txt
- 1.87.0
+ 1.104.0