fix: add MaxHeaderSize guard to HeaderScanner

[lxcxjxhx]

What

Add a MaxHeaderSize field to HeaderScanner. When set to a positive value, the scanner stops parsing and returns errHeaderTooLong once the cumulative header bytes exceed the limit. The default is 0 (no limit), so existing callers are unaffected.

Why

Malicious or buggy clients can send an arbitrarily large header block and cause the parser to loop until OOM. Having a built-in size guard lets callers defend against this without wrapping the scanner externally.

Changes

• headerscanner.go: add MaxHeaderSize int field and a size check at the top of Next()
• headerscanner_test.go: add TestHeaderScannerMaxHeaderSize and TestHeaderScannerMaxHeaderSizeDisabled

Testing

go test ./pkg/protocol/http1/ext/...

All existing tests pass. New tests cover:

• Headers within limit → parsed normally
• Headers exceeding limit → errHeaderTooLong
• MaxHeaderSize=0 → no limit (backward compatible)

Closes #1497

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