feat(clients): add native Gemini API client

[TigerInYourDream]

Summary

Adds a native Gemini API client (GeminiClient) that talks directly to Google's Generative Language API, without going through an OpenAI-compatible shim.

Changes

• src/clients/gemini.rs (new) — Full BotClient implementation for Gemini:

  • Model listing via GET /models with generateContent capability filtering
  • Streaming chat completions via :streamGenerateContent with SSE
  • Proper role mapping: User/Tool → user, Bot → model, System → system_instruction
  • API key auth via x-goog-api-key header + key query param
  • Robust URL building that preserves existing base-URL query params
  • 5 unit tests covering model parsing, URL construction, request building, and stream parsing

• src/clients.rs — Registers the gemini module under the api-clients feature gate

• src/prelude.rs — Re-exports GeminiClient for ergonomic usage

Design Notes

• Follows the same Arc<RwLock<Inner>> pattern used by OpenAiClient for thread-safe cloning
• Uses async_stream + parse_sse (existing crate utility) for streaming
• All async work returns futures — no internal task spawning, consistent with AITK's design philosophy
• Cross-platform compatible (no tokio-specific APIs in the client itself)

[TigerInYourDream]

This incremental update includes 3 main changes:

1. fix(sse): parse CRLF events and skip non-data frames safely (e94d05b)

   • Fixed SSE parsing compatibility for \r\n\r\n event separators.
   • Safely skips non-data: frames to avoid Option::unwrap() panics.

2. feat(gemini): implement native tool-calling roundtrip (76c9e54)

   • Implemented full Gemini native tool-calling roundtrip (model emits function call -> local tool execution -> function response returned -> model continues and produces final answer).
   • Aligned request/response behavior with Gemini native fields and tool mode constraints.

3. feat(example): add gemini native tool-calls demo (49d71cc)

   • Added a runnable example at examples/gemini-tool-calls to verify native Gemini tool-calling behavior.

Validated: the example reliably triggers get_weather tool calls and returns the final answer, which confirms the native tool-calling path in this PR is working.

[TigerInYourDream]

This incremental update includes 3 main changes:

1. fix(sse): parse CRLF events and skip non-data frames safely (e94d05b)

   • Fixed SSE parsing compatibility for \r\n\r\n event separators.
   • Safely skips non-data: frames to avoid Option::unwrap() panics.

2. feat(gemini): implement native tool-calling roundtrip (76c9e54)

   • Implemented full Gemini native tool-calling roundtrip (model emits function call -> local tool execution -> function response returned -> model continues and produces final answer).
   • Aligned request/response behavior with Gemini native fields and tool mode constraints.

3. feat(example): add gemini native tool-calls demo (49d71cc)

   • Added a runnable example at examples/gemini-tool-calls to verify native Gemini tool-calling behavior.

Validated: the example reliably triggers get_weather tool calls and returns the final answer, which confirms the native tool-calling path in this PR is working.

[TigerInYourDream]

[TigerInYourDream]

Changes since last review

Core: stream tool-call id collision fix

The previous implementation used stream_index alone to correlate tool calls across SSE chunks. When indices shift between events (e.g. chunk 1 sends A at index 0 and B at index 1, chunk 2 resends only B at index 0), the old code would misassign A's fallback id to B's protocol id.

Promotion policy (3-tier):

1. Same stream_index + same signature → promote immediately (strongest correlation, no ambiguity)
2. Global signature search with exactly one candidate → promote (unambiguous even after index shift)
3. Global signature search with 0 or 2+ candidates → don't promote, insert as new entry

The key design choice is tier 3: when multiple fallback calls share the same signature (same function name + args) and none has a protocol id yet, the protocol layer cannot tell them apart. Rather than guessing and potentially renaming the wrong call (state corruption), we conservatively skip the promotion. This may produce a duplicate call, but a duplicate is always safer than a wrong-assignment — downstream tool execution can handle an extra call, but cannot recover from a call whose id was silently swapped.

This limitation is inherent to the Gemini streaming protocol when functionCall.id is absent. The ambiguity resolves naturally once Gemini provides protocol ids for all calls.

Other changes

• as_tool_parts now returns Result and errors on unknown tool_call_id instead of silently degrading to a text part
• Fixed ensure_ordered_id checking wrong collection (calls_by_id → order)
• Collapsed nested if let per clippy, tightened comments, trimmed redundant tests (19 → 12)