fix(cache): deterministic Responses ids, stable session anchor, structural Anthropic breakpoints

[lecommander]

Summary
Reduce provider prompt-cache misses caused by client-side prefix instability. Three independent fixes, each verified by the existing suite plus two new regression tests. No behavior change for cache-ineligible routes.

Changes

• Responses item ids are now deterministic (P0a): responsesFunctionCallItemId maps non-fc ids to fc + sha256(call id, 32 hex) instead of randomUUID per request, fc ids pass through verbatim. Same history now replays byte-identically every turn. Previously the prefix cache broke at the first function_call item on every tool turn (same class as OpenHands SDK 2905).
• Stable session anchor (P0b): the x-opencode-session fallback no longer hashes the first 3 messages (messages 1-2 shift on every history trim by design). It now hashes first message + sorted tool names + model id, memoized per conversation for its lifetime. Legacy conversationAnchor kept for compatibility.
• Structural Anthropic breakpoints (P1): buildAnthropicMessages placed cache_control on the first 4 content blocks in emit order, so any added text/image part shifted all later marks and consumed the 4-slot budget gateways also inject into. Now at most 2 marks at structural boundaries: end of message 0 (static prefix) and end of message N-2 (tail checkpoint); only the last block of a marked message carries the mark.

Before / After

• Before: Responses replay emitted a fresh fc id per request, session id re-hashed over shifting messages, Anthropic marks drifted with content growth.
• After: replay payloads are deterministic, session header is constant across trims, Anthropic marks sit on stable boundaries. DeepSeek-class implicit routes get stability with zero new markers.

Verification

• npx tsc -p ./ clean
• Targeted: 24/24 pass (responsesRequest, session-header, headers)
• Full npm test: 497/497 pass
• Live gate to run after install: 30-turn run, prefix hash and session constant, cacheHit at 98 percent plus after turn 2, exactly one dip across a trim event.

Notes

• Explicit markers remain gated to Anthropic-native and GPT-5.6-family routes by follow-up work; DeepSeek/Qwen/Kimi/GLM/MiniMax/MiMo/Muse stay marker-free (implicit only).
• No keepalive, no previous_response_id chaining, no 1h TTL stamp in this PR.
• Draft status: asking for direction on breakpoint policy and per-route gating before packaging a vsix.