feat: context-based fixture routing via X-AIMock-Context header

[jpr5]

Summary

• Adds X-AIMock-Context header support for scoping fixtures per integration — fixtures with match.context only match requests carrying that exact context value; fixtures without context remain shared
• Injects context extraction in all 21 handler files (28 call sites) — HTTP handlers via getContext(req), WebSocket handlers via upgradeHeaders extraction
• Enriches recorder to auto-capture context in buildFixtureMatch() and route recorded fixtures into context subdirectories in persistFixture()
• Adds context validation in fixture-loader (type check, dedup key, discriminator)
• 15 new tests across 5 test files covering matching, helper extraction, fixture loading, recording, and strict-mode interaction
• Bumps version to 1.26.0 (includes timing-replay from Timing-aware recording and replay with --replay-speed multiplier #223)

Test plan

• 3151 tests pass (90 files)
• TypeScript clean (tsc --noEmit — 0 errors)
• Prettier clean
• ESLint clean
• Pre-commit hooks pass on all commits
• CR loop: 7-agent Round 1 + confirmation round — 0 bucket (a) findings on context routing code

[pkg-pr-new]

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npm i https://pkg.pr.new/@copilotkit/aimock@226

commit: 754c48b