test(client): expand model-based tests with 409, empty 200, missing headers

Adds 3 new command types to the fast-check model-based property test:
- Respond409: verifies 409 (shape rotation) doesn't affect retry counter
- Respond200Empty: verifies empty message batch doesn't reset counter
- RespondMissingHeaders: verifies non-retryable errors terminate immediately

Also fixes cross-iteration cache pollution (expiredShapesCache, localStorage)
and tracks handle rotation so 409 → subsequent response sequences are
protocol-correct. Bumped to 200 runs with 8 command types.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: KyleAMathews