Log pathway search, click, and hover analytics to Mixpanel (SCP-5989)

[eweitz]

This tracks user interactions with new pathway exploration features introduced in #2206. It helps assess usage.

Previously, queries from the main search box in Study Overview were all logged as "gene" searches. Interactions with the pathway diagram were also unlogged. We had very little observability on user engagement for this significant new feature.

Now, if a user clicks on one of the new pathway suggestions in the search autocomplete, then the search is logged as a "pathway" search. Hover / mouseover on gene nodes in the pathway diagram, and click on pathway nodes, are also now logged.

Test

A new automated test verifies some new logging. To optionally manually test:

1. Go to "Cellular and transcriptional diversity over the course of human lactation" study
2. Open Network panel in Chrom DevTools, filter to "event method:POST"
3. In search box, type "lac"
4. Select first pathway suggestion
5. Confirm an event request is posted to Mixpanel with name "search" and type "pathway"
6. Hover over a gene node in the pathway diagram
7. Confirm event request with name "pathway-node-tooltip"
8. Click on a green pathway node
9. Confirm event request with name "pathway-node-click"

This satisfies SCP-5989.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 71.12%. Comparing base (def2112) to head (c75047a).

Files with missing lines	Patch %	Lines
...pp/javascript/components/visualization/Pathway.jsx	0.00%	5 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           ew-refine-pathways    #2249      +/-   ##
======================================================
- Coverage               71.13%   71.12%   -0.01%     
======================================================
  Files                     335      335              
  Lines                   28671    28680       +9     
  Branches                 2522     2523       +1     
======================================================
+ Hits                    20395    20399       +4     
- Misses                   8134     8139       +5     
  Partials                  142      142              

Files with missing lines	Coverage Δ
app/javascript/lib/search-metrics.js	63.38% <100.00%> (+1.06%)	⬆️
...pp/javascript/components/visualization/Pathway.jsx	64.36% <0.00%> (-3.93%)	⬇️

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