feat: Add error cause detection

[metal-messiah]

Detect and report the cause attribute on captured JavaScriptError events. If the cause is an instance of an Error, the stack trace will be captured, otherwise the stringified contents of the cause will be reported. This will be queryable in JavaScriptError events under the attribute cause.

Overview

Related Issue(s)

https://new-relic.atlassian.net/browse/NR-414116?atlOrigin=eyJpIjoiODY2NDUxZTIwMTUxNGQ0YWFkMDU0M2JmMDkxNzBkOWYiLCJwIjoiaiJ9

Testing

New tests have been added to validate the various cause cases.

[github-actions]

Supportability Metric Usage Changes:

No matching changes found

Supportability Metrics .md File Changes:

supportability_metrics.md was changed? false

New supportability metrics require changes to supportability_metrics.md, as well as a new PR to Angler. Please ensure an Angler PR is created and linked to this PR.

[github-actions]

Asset Size Report

Merging this pull request will result in the following asset size changes:

Agent	Asset	Previous Size	New Size	Diff
lite	loader	25.57 kB / 9.64 kB (gzip)	25.57 kB / 9.64 kB (gzip)	0% / 0% (gzip)
lite	async-chunk	54.16 kB / 17.89 kB (gzip)	54.16 kB / 17.89 kB (gzip)	0% / 0% (gzip)
pro	loader	53.74 kB / 18.88 kB (gzip)	53.75 kB / 18.89 kB (gzip)	0.03% / 0.03% (gzip)
pro	async-chunk	92.72 kB / 28.71 kB (gzip)	92.93 kB / 28.8 kB (gzip)	0.23% / 0.3% (gzip)
spa	loader	62.5 kB / 21.6 kB (gzip)	62.51 kB / 21.61 kB (gzip)	0.03% / 0.05% (gzip)
spa	async-chunk	117.3 kB / 35.7 kB (gzip)	117.51 kB / 35.78 kB (gzip)	0.18% / 0.22% (gzip)

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.78%. Comparing base (42a16ee) to head (5dbe4cb).

Files with missing lines	Patch %	Lines
src/features/jserrors/aggregate/index.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1531      +/-   ##
==========================================
- Coverage   87.79%   87.78%   -0.02%     
==========================================
  Files         198      199       +1     
  Lines        7808     7817       +9     
  Branches     1573     1577       +4     
==========================================
+ Hits         6855     6862       +7     
- Misses        822      824       +2     
  Partials      131      131              

Flag	Coverage Δ
unit-tests	79.08% <80.00%> (-0.01%)	⬇️

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[github-actions]

Last ran on July 24, 2025 13:02:37 CDT
Checking merge of (5dbe4cb) into main (42a16ee)