[Repo Assist] Fix #341: IFsiEvaluator inherits IDisposable; FsiEvaluator disposes FSI session

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Closes #341.

Summary

IFsiEvaluator did not inherit IDisposable, so the underlying F# interactive session started by FsiEvaluator was never disposed. Since a new FsiEvaluator is created for each .fsx file processed by fsdocs build, this caused a session leak on every literate script file evaluation.

Changes

• IFsiEvaluator now inherits System.IDisposable (breaking change — custom implementations must add Dispose())
• FsiEvaluator implements IDisposable by forwarding to fsiSession.Dispose()
• BuildCommand.fs wraps ParseAndTransformScriptFile in try/finally to guarantee the FSI session is disposed after each .fsx file is processed, even if evaluation throws
• Updated call sites to use new FsiEvaluator(...) syntax as recommended for IDisposable types

Breaking Change Note

Adding inherit IDisposable to IFsiEvaluator is a breaking change for any user who has implemented the interface directly. The more common usage — constructing and using FsiEvaluator — is unaffected. This change should be noted in the release notes as a minor breaking addition.

Test Status

Passed!  - Failed: 0, Passed: 101, Skipped: 0, Total: 101  (FSharp.Literate.Tests)
Passed!  - Failed: 0, Passed: 72, Skipped: 4, Total: 76    (FSharp.ApiDocs.Tests)
Build succeeded. 0 Errors, 0 Warnings (FS0760 resolved).

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