fix(claude,gemini,antigravity): centralize tool-call ID generation and harden signature sanitization

- Add shared `GenerateClaudeToolCallID` in `common` and replace duplicated local `toolu_` generators across Claude request translators for consistent, uniform ID generation.
- Replace `bufio.Scanner`-based SSE line handling with manual newline parsing and use `JoinRawArray` for array assembly to avoid scanner buffer limits and extra wrapping allocations.
- Improve carrier/thought signature scrubbing to safely strip internal metadata only when present and detect spoofed/duplicate `thoughtSignature` keys (including raw/unicode-escaped forms) during sanitized JSON traversal.