feat: add continuation diagnostics

[HisenWeb]

Summary

Adds P1-4 diagnostics around Codex tool-result continuation and stored-state recovery.

This is logging/documentation-only. It does not change protocol behavior, state lookup rules, fallback behavior, or Responses output shape.

Changes

• Logs stateless_tool_history_bypass_state_lookup when full stateless history bypasses stored-state lookup.
• Logs stored_response_not_found when a requested stored response ID is missing from non-expired state.
• Logs tool_history_unique_fallback_hit when continuation is recovered via unique pending call_id fallback.
• Logs tool_history_call_id_ambiguous and tool_history_response_ambiguous when fallback is refused due ambiguity.
• Logs tool_history_call_id_not_found when fallback cannot match all requested call IDs.
• Adds docs/p1-continuation-diagnostics.md with event names and operational interpretation.

Validation

Edited via GitHub connector. Please run locally before merge:

cargo fmt --check
cargo test --lib
cargo test

Notes

I intentionally avoided modifying server.rs; diagnostics were added in the narrow conversion/state layers that own the relevant decisions.

[chatgpt-codex-connector]

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