Add native Google provider config and doctor support

[lazyeo]

Summary

• adds the native Google/Gemini provider path in the core provider layer
• wires provider: google through project config, global config, init scaffolding, and doctor/config help text
• supports Gemini-native tool/function calling turns, including replaying tool results and preserving thoughtSignature across the loop
• updates README docs (zh/en/ja) to show native Google usage and the current recommended default model

What was validated online

Validated against the live Google native API:

• non-stream text generation via generateContent
• Gemini-native function/tool calling
• replaying tool results back into Gemini-native turns
• preserving and forwarding thoughtSignature in tool-call replay
• Gemini 2.5 models as the current recommended path, plus 3.x preview models as validated preview options

Current recommendation

• default stable recommendation: gemini-2.5-flash
• gemini-3.x preview models were validated, but are not presented as the default stable recommendation in docs/help/config scaffolding

Current boundaries

This PR documents and ships the validated Google-native scope above. It does not claim blanket provider parity beyond what was exercised.

Known boundary notes:

• the validated path is focused on native generation + native tool-calling loops
• preview Gemini 3.x models remain preview choices
• any broader parity claims should wait for additional targeted validation

[lazyeo]

Quick update: this branch has been rebased onto the latest upstream master (currently aligned with the v1.1.1 line), and the Google provider commits were replayed cleanly.

Post-rebase, the Google-provider-related core tests still pass locally, and the branch remains focused on:

• native Google provider config + doctor support
• native Gemini tool calling
• thoughtSignature preservation during tool replay
• docs/help/init guidance for the Google native path