Validate HttpVersionPolicy enum values on setters

[Copilot]

HttpRequestMessage.VersionPolicy and HttpClient.DefaultVersionPolicy accepted arbitrary HttpVersionPolicy values, deferring failures to deep inside the request pipeline (or silently mis-behaving).

Changes

• HttpRequestMessage.VersionPolicy setter — reject values outside RequestVersionOrLower..RequestVersionExact with ArgumentException (paramName value).
• HttpClient.DefaultVersionPolicy setter — same validation, before the existing CheckDisposedOrStarted() call.
• Error message — reuses the existing SR.net_invalid_enum resource ("The specified value is not valid in the '{0}' enumeration."), matching the pattern already used in System.Net.Security (SslClientAuthenticationOptions, SslStream, etc.).
• Tests — added theory tests covering -1, 3, and int.MaxValue for both setters.

public HttpVersionPolicy VersionPolicy
{
    get => _versionPolicy;
    set
    {
        if ((uint)value > (uint)HttpVersionPolicy.RequestVersionExact)
        {
            throw new ArgumentException(SR.Format(SR.net_invalid_enum, nameof(HttpVersionPolicy)), nameof(value));
        }
        CheckDisposed();
        _versionPolicy = value;
    }
}

[Copilot]

Pull request overview

This PR tightens argument validation for HttpVersionPolicy setters in System.Net.Http so invalid enum values fail fast at the API boundary rather than later in the request pipeline.

Changes:

• Added range validation to HttpRequestMessage.VersionPolicy setter, throwing ArgumentException for out-of-range HttpVersionPolicy values.
• Added the same validation to HttpClient.DefaultVersionPolicy setter.
• Added functional tests covering several invalid enum values for both setters.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Net.Http/src/System/Net/Http/HttpRequestMessage.cs	Validates VersionPolicy is within the defined HttpVersionPolicy range before assignment.
src/libraries/System.Net.Http/src/System/Net/Http/HttpClient.cs	Validates DefaultVersionPolicy is within the defined HttpVersionPolicy range before CheckDisposedOrStarted() and assignment.
src/libraries/System.Net.Http/tests/FunctionalTests/HttpRequestMessageTest.cs	Adds theory coverage verifying invalid VersionPolicy values throw ArgumentException with paramName value.
src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientTest.cs	Adds theory coverage verifying invalid DefaultVersionPolicy values throw ArgumentException with paramName value, and client remains usable after.

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