Remove feature flag for transitive #: directives

[jjonescz]

Follow up on #53775 (comment).

[Copilot]

Pull request overview

Removes the feature-flag gating for processing transitive file-level #: directives in file-based programs, aligning implementation, tests, and docs with the new default behavior.

Changes:

• Stop checking ExperimentalFileBasedProgramEnableTransitiveDirectives when validating directives (transitive directives always allowed).
• Update dotnet run file tests to no longer set the transitive-directives flag and remove the flag-specific test.
• Remove the flag constant from the FileBasedPrograms internal API surface and update documentation accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/dotnet.Tests/CommandTests/Run/RunFileTests.cs	Removes transitive-directive flag setup and deletes the test that asserted the flag gating behavior.
src/Microsoft.DotNet.ProjectTools/VirtualProjectBuilder.cs	Removes the runtime check that required the transitive-directives flag for non-entry-point directives.
src/Cli/Microsoft.DotNet.FileBasedPrograms/InternalAPI.Unshipped.txt	Drops the internal API entry for the removed flag constant.
src/Cli/Microsoft.DotNet.FileBasedPrograms/FileLevelDirectiveHelpers.cs	Removes the transitive-directives flag constant from CSharpDirective.IncludeOrExclude.
documentation/general/dotnet-run-file.md	Removes documentation that stated transitive directives were gated behind a feature flag.

[DamianEdwards]

Are we going to get this into 10.0.300 as well?

[jjonescz]

IIRC, code complete of 10.0.3xx was yesterday. Also the IDE integration is probably not even released yet.

[DamianEdwards]

We should check on the code complete status and if there's a process for getting changes in still. On the IDE side they release far more often and the experience in the insiders and pre-release versions is already good and we can do follow ups to tweak the experience as needed. Is the opt-in flag for transitive directives removed for .NET 11 already?

[jjonescz]

Is the opt-in flag for transitive directives removed for .NET 11 already?

No, but this PR will flow there once merged.