fix: make PagingLogger IDisposable to prevent fd leak on step fault

[herbenderbler]

Summary

PagingLogger can lose partial step logs and hold file handles open when a step faults before ExecutionContext.Complete() reaches _logger.End().

This PR makes logger lifetime explicit: owned PagingLogger instances are disposed when an ExecutionContext completes, while shared embedded loggers are not double-disposed.

What Changed

• Keep PagingLogger disposable behavior and focused disposal tests in src/Test/L0/PagingLoggerL0.cs.
• Add ownership tracking in src/Runner.Worker/ExecutionContext.cs:
  • mark loggers created via HostContext.CreateService<IPagingLogger>() as owned
  • dispose owned loggers in Complete() via finally
  • avoid disposing shared embedded loggers
• Add ExecutionContext L0 tests in src/Test/L0/Worker/ExecutionContextL0.cs to verify:
  • owned job logger is disposed
  • owned child logger is disposed
  • shared embedded logger is not disposed
• Trim low-signal/diagnostic-only F4 tests to keep the suite focused and maintainable.

Why This Is Safer

• Prevents leaked file handles from owned loggers on non-happy paths.
• Reduces risk of missing partial logs by ensuring disposal runs deterministically when contexts complete.
• Preserves existing shared-logger behavior for embedded execution contexts.

Test Plan

• Existing L0 PagingLogger disposal behavior tests pass in unit test environment.
• New L0 ExecutionContext ownership tests cover the production ownership path.
• git diff --check and IDE lints are clean.

[Copilot]

Pull request overview

Updates PagingLogger to be disposable so file handles are deterministically released (and partial logs are still queued) when a step faults before End() is reached, preventing fd leaks and silent log loss.

Changes:

• Extend IPagingLogger with IDisposable and implement PagingLogger.Dispose() (best-effort, idempotent, non-throwing).
• Add L0 tests covering the new disposal contract and expected behavior on exception-like paths.
• Add a PR write-up document; update .gitignore.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/Runner.Common/Logging.cs	Makes IPagingLogger disposable and adds PagingLogger.Dispose() to flush/close and queue partial uploads safely.
src/Test/L0/PagingLoggerL0.cs	Adds new L0 tests validating IDisposable contract and disposal behavior (partial write, idempotence, using-pattern, etc.).
PR_DESCRIPTION_F4.md	Documents the bug, repro, and rationale for the IDisposable fix and tests.
.gitignore	Modifies ignore rule related to DotSettings / leakbench artifacts.