refactor(proxy): replace panic-prone unwrap/expect with safe patterns

Harden the proxy module against panics by removing optimistic
unwrap()/expect() calls in favor of pattern matching and graceful
fallbacks:

- copilot_optimizer/cache_injector: bind Value::Array/String directly
  instead of is_array()+as_array().unwrap(); use is_none_or and in-place
  string mutation
- hyper_client: gate the raw-write path with if-let + filter instead of
  has_cases + unwrap()
- gemini_shadow: recover poisoned RwLock via into_inner() rather than
  panicking, with warn logging
- streaming_codex_chat: replace expect("tool state exists") with
  let-else (return/continue)
- merge_tool_results: early-return when messages is absent
- sse: fall back to from_utf8_lossy on the UTF-8 boundary slice

No behavior change on the happy path; all proxy tests pass.