fix(proxy): support explicit opus 4.6 and 4.7 pins

[briankeefe]

Summary

• catch Claude Code's shorter "You're out of extra usage" error so 1M requests fall back to the base model instead of surfacing a 500
• keep claude-opus-4-7 as Meridian's default Opus pin for bare opus requests
• preserve explicit claude-opus-4-6 and claude-opus-4-7 requests by passing a per-request SDK env override that survives the 1M fallback retry
• when a resumed Opus session still gets the same extra-usage error after falling back to base opus, evict the cached session and retry once from a fresh prompt on the same model

Testing

• bun test src/__tests__/proxy-extra-usage-fallback.test.ts src/__tests__/session-fingerprint-context.test.ts src/__tests__/errors.test.ts
• bun test src/__tests__/errors.test.ts src/__tests__/proxy-extra-usage-fallback.test.ts src/__tests__/proxy-env-stripping.test.ts src/__tests__/models.test.ts src/__tests__/query.test.ts

[rynfar]

sorry for the delay ive been traveling, ill try to get to this today

[rynfar]

LGTM. Tight scope, mirrors existing retry patterns, comprehensive test coverage. Verified 1738/1738 tests pass locally and CI is green. Regression risk on non-opus and aliased paths is nil (envOverrides only set for explicit claude-opus-* requests). Fresh-retry is properly guarded by didYieldContent/didYieldClientEvent against post-yield retries.