fix: preserve MCP JSON Schema references in Gemini tool declarations

[chenyme]

Summary

Fixes MCP tool schemas losing $ref branches when converted for Gemini.

Generate Content now uses the official parametersJsonSchema field, while Gemini Interactions passes the original JSON Schema directly. This preserves array items, property references, anyOf, and $defs without custom dereferencing or compatibility code.

Change type

• Bug fix

Affected areas

• Backend / API
• Conversations / streaming
• MCP / tools

Verification

• cd backend && go test ./... -count=1
• cd backend && go test -race ./internal/infra/llm -count=1
• cd backend && go vet ./...
• cd backend && make build
• git diff --check

Screenshots, API examples, or logs

Not applicable.

Configuration, migration, and compatibility notes

• No configuration or database changes.
• No migration required.
• No API contract changes outside Gemini tool declaration payloads.
• MCP schemas are passed through using the current official Gemini JSON Schema fields.
• External $ref URLs are not resolved or fetched.

Documentation

• Documentation is not needed for this change.

Security and privacy

• No secrets, tokens, credentials, or personal data are included.
• User data access remains scoped by authenticated user context.
• No external schema references are fetched.
• Security-sensitive behavior was reviewed.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests and static verification were run.
• User-facing behavior and API compatibility impact were reviewed.
• Generated artifacts are not included.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.