Mirror Windows behavior for managed HttpListener Content-Length

[iremyux]

This PR aligns managed HttpListener Content-Length parsing behavior with Windows behavior by rejecting out-of-range numeric values instead of treating certain overflow values as zero.

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[Copilot]

Pull request overview

This PR changes the managed (System.Net.Managed) HttpListener request header parsing to treat out-of-range Content-Length values as invalid (i.e., parse failure) rather than accepting them and normalizing certain overflow cases to 0, aligning the managed behavior with the Windows implementation’s strict decimal parsing shape.

Changes:

• Updated managed Content-Length parsing to use long.TryParse(..., NumberStyles.None, InvariantCulture, ...) and reject parse failures.
• Moved coverage for overflow Content-Length values from “property returns 0” semantics to “invalid client request” semantics (400 Bad Request).
• Adjusted tests to keep the valid upper bound (long.MaxValue) case while removing the previous “overflow => 0” expectations.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Net.HttpListener/tests/InvalidClientRequestTests.cs	Adds invalid-request cases asserting Bad Request for Content-Length values beyond long.MaxValue.
src/libraries/System.Net.HttpListener/tests/HttpListenerRequestTests.cs	Removes prior expectations that overflow Content-Length values are treated as 0.
src/libraries/System.Net.HttpListener/src/System/Net/Managed/HttpListenerRequest.Managed.cs	Switches to strict long.TryParse-based Content-Length parsing and rejects out-of-range values.