Improve runner tracking

[mvanderkamp]

This PR includes several fixes to improve support for tracking the runner across windows and panes:

• Select the correct session, window, and pane (if applicable) when selecting the runner, not just the correct window/pane.
• Enter copy-mode in the correct window and/or session when inspecting the runnner.
• Specify an end-of-word pattern when checking for runner existence to make sure we match the whole ID.
  • e.g. If the runner index was '%15' it previously would have reported that the runner exists even if it doesn't but a pane with index e.g. '%157' existed.
• To help with debugging, a VimuxHasRunner() function has been added.

[Copilot]

Pull Request Overview

This PR refactors tmux command invocations to be more explicit and fixes a runner detection bug. The changes make the code more robust when working with both pane and window runner types.

• Simplified VimuxTogglePane() to always use last-pane instead of dynamically choosing between pane/window
• Refactored VimuxZoomRunner() and VimuxInspectRunner() to work consistently for both pane and window types by explicitly switching client and selecting window/pane
• Added public VimuxHasRunner() function to expose runner status
• Fixed s:hasRunner() to use word boundary matching to prevent false positives in runner detection

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