fix(security): reject underscore HTTP headers (closes #578)

[s2x]

Description

Closes #578.

RequestConverter previously converted both dash and underscore header names to the same HTTP_* server key. An attacker could use X-Forwarded_For to override a trusted proxy's X-Forwarded-For value or bypass proxy header stripping.

Changes

• Discard header names containing _ before constructing Symfony's server bag.
• Log each dropped header name once per worker at warning level.
• Preserve dash-spelled headers and CGI CONTENT_TYPE, CONTENT_LENGTH, and CONTENT_MD5 handling.
• Add trusted-proxy, header-stripping, logging, documentation, changelog, and benchmark coverage.

Verification

• composer lint passed.
• composer test passed: 1583 tests, 13378 assertions, 18 skipped.
• vendor/bin/phpbench run benchmarks/RequestConverterBench.php --report=aggregate passed.
• composer test:coverage could not run the coverage gate because this environment has no PCOV/Xdebug driver.

Code Review

• Reviewed the final diff for security, edge cases, style, and test coverage.
• Addressed local lint/static-analysis findings.