feat(llmhubs): support tool calling and structured output in the Gemi…

[xinkuleee]

Summary

Adds Google Gemini function/tool calling and JSON-schema structured output to the
LLMHub Gemini adapter, so Gemini models can drive the chat agent (tools) and
structured routing (intent check).

• Tool calling: map internal (OpenAI-shaped) tools / function_call /
  function_result to Gemini functionDeclarations / functionCall /
  functionResponse (role mapping assistant→model / tool→user, and
  call_id↔function-name correlation since Gemini responses carry no call id);
  parse Gemini functionCall back into function_call outputs.
• Schema sanitizer: rewrite tool/response JSON Schema into Gemini's
  OpenAPI-subset — inline $ref/$defs, drop additionalProperties/$schema,
  flatten single-element allOf, and convert Optional anyOf + {"type":"null"}
  to nullable.
• Structured output: map response_format.type=json_schema to Gemini
  responseMimeType + responseSchema (fixes chat intent-check routing on Gemini).
• Capability flags: advertise supports_tools / supports_structured_output
  for the Gemini provider (core io_profile derivation + backend equivalent).
• Tests: unit coverage for request building, input/response mapping, the
  sanitizer (refs/defs/additionalProperties/allOf/anyOf-null/recursion), and
  structured output.

Validation

• make precommit-run
• cd backend && go test ./...
• cd core && uv run pytest
• Frontend build/lint, if working from a frontend source checkout
• Not run; explain below

Frontend is untouched, so its build/lint does not apply.

Checklist

• Linked relevant issues or explained why there is no issue.
• Updated docs and examples where behavior changed.
• Updated CHANGELOG.md under ## [Unreleased] for user-facing changes.
• Regenerated and committed generated files after proto, Wire, or OpenAPI changes.
• Confirmed no secrets, tokens, credentials, or sensitive logs are included.

Notes for reviewers

• Streaming is not implemented for Gemini yet — it falls back to the base
  non-streaming path (generate_stream yields a single chunk). Tool calling works,
  but without incremental token streaming; a follow-up can add
  streamGenerateContent?alt=sse.
• The schema sanitizer targets the keywords Gemini currently rejects; verified
  against the full builtin tool set and the structured intent-check schema.
• No secrets/config included: the local Gemini model YAML (with API key) and the
  local default-model switch are gitignored and intentionally excluded from this PR.