test: add missing HTTP analyzer test coverage

[Zious11]

Summary

Test coverage review identified 7 gaps at criticality >= 7/10:

1. Garbage input recovery — send binary data, verify buffer cleared and subsequent valid HTTP works
2. Buffer cap (64KB) — verify headers > 64KB don't cause unbounded growth or panic
3. Long URI detection — test URI > 2048 chars triggers Execution finding
4. Empty User-Agent detection — test empty vs missing UA distinction
5. Admin panel detection — test /wp-admin, /phpmyadmin patterns
6. Partial response reassembly — split response across chunks
7. Multiple concurrent flows — verify flow isolation (different FlowKeys)

Context

Found in HTTP analyzer test coverage review.