Fix plugin cache panic when cwd is unavailable

[etraut-openai]

Summary

Fixes #16637. (I hit this bug after 11h of work on a long-running task.)

Plugin cache initialization could panic when an already-absolute cache path was normalized through AbsolutePathBuf::from_absolute_path, because that path still consulted current_dir().

This changes absolute-path normalization so already-absolute paths do not depend on cwd, and makes plugin cache root construction available as a fallible path through PluginStore::try_new(). Plugin cache subpaths now use AbsolutePathBuf::join() instead of re-absolutizing derived absolute paths.

[xl-openai]

Stamp to unblock, but I wonder if the root cause (current_dir() being unavailable) affects more than just plugins. Are we okay with that state?

[etraut-openai]

Yes, unavailable cwd can surface elsewhere, but this PR covers the accidental dependency on cwd for already-absolute paths. I think we should be resilient to a panic in this case. The plugin code was a victim of this bug in absolutize(). The fix in that function is the most important part of this PR.

I also added additional hardening and error reporting specifically for the plugin code. That's less important, but it's useful for diagnosing any issue related to invalid plugin cache paths.