fix: reject malformed content-length request headers

[trivikr]

This relates to...

Fixes: #5059

Rationale

content-length was being validated too loosely because malformed values could be partially normalized before later request-body length checks.

This change makes the header validation stricter so only digit-only values are accepted during request parsing.

Changes

• Reject non-string, empty, fractional, signed, and partially numeric content-length values before parsing them.
• Preserve existing behavior for valid decimal integer content-length values.

Features

N/A

Bug Fixes

Request content-length malformed values are rejected with InvalidArgumentError('invalid content-length header') instead of being normalized or deferred into a later mismatch error.

Breaking Changes and Deprecations

Requests that previously accepted malformed content-length values will now fail immediately during header validation.

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.09%. Comparing base (a1d6766) to head (46a5113).

Files with missing lines	Patch %	Lines
lib/core/request.js	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5060      +/-   ##
==========================================
- Coverage   93.10%   93.09%   -0.01%     
==========================================
  Files         110      110              
  Lines       35770    35788      +18     
==========================================
+ Hits        33303    33318      +15     
- Misses       2467     2470       +3     

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[mcollina]

lgtm