fix(http): prevent cached request header leakage (closes #576)

[s2x]

Description

Closes #576

Changes

• Restore parsed request headers after every handler dispatch, including when the handler throws.
• Prevent middleware mutations from persisting in Workerman's request cache.
• Add regression coverage for normal and exceptional handler paths.
• Update security documentation and changelog.

Verification

• composer lint
• composer test
• Code review completed; no blocker findings

Code Review

• Passed subagent code review
• All review comments addressed