Add analytics instrumentation for file tree browser

What does this MR do and why?

This is part of a bigger effort to implement a file tree browser (FTB) in the repository. All changes are behind repository_file_tree_browser FF.

Specifically, this MR adds 5 instrumentation events.

⚠️ Metrics will be added in a follow up.

References

[instrumentation plan] - file tree browser (#562718 - closed)

Screenshots or screen recordings

Event Name	Description	Trigger Method	Additional Properties	Video
click_file_list_on_repository_page	User clicks a row within the main file list	Click event on <tr class="tree-item">	None	CleanShot 2025-09-02 at 14.57.52.mp4
click_file_tree_browser_on_repository_page	User clicks a row within the file tree browser	Click event in clickFile() method	None	CleanShot 2025-08-29 at 17.42.39.mp4
expand_file_tree_browser_on_repository_page	User expands the file tree browser	Button click or keyboard shortcut (shift+ f key)	label: 'click' or label: 'shortcut'	CleanShot 2025-08-29 at 17.40.53.mp4
collapse_file_tree_browser_on_repository_page	User collapses the file tree browser	Button click or keyboard shortcut (shift+ f key)	label: 'click' or label: 'shortcut'	see above
focus_file_tree_browser_filter_bar_on_repository_page	User focuses the file tree browser filter bar	Click on filter input or keyboard shortcut (f key)	label: 'click' or label: 'shortcut'	CleanShot 2025-08-29 at 17.41.44.mp4

Event Details:

• Identifiers: All events track project, namespace, and user
• Product Group: source_code
• Product Category: source_code_management
• Milestone: 18.4
• Tiers: Available on free, premium, and ultimate

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Related to #562718 (closed)