Migrate unit tests to a dedicated cloud build private pool

[linsword13]

We've been seeing more internal errors from the regional (us-central1) default pool, so migrating to use a private pool for these tests. We pick southamerica-west1 as we have the quota.

[gemini-code-assist]

Code Review

This pull request refactors the Google Cloud Build and Terraform configurations to consolidate worker pools under a unified private pool (n2d_pool). Feedback highlights two main issues: first, changing the physical name of the worker pool resource will force a resource replacement (destroy and recreate) in GCP, which can be avoided by keeping the physical name while refactoring the Terraform address; second, setting the default value of _WORKER_POOL to an empty string in the unit tests configuration will cause Cloud Build validation errors if the substitution is not overridden.

[ramble-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: 3c11668

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	2.0809	2.0776 (2d42cdb)	2.0633
test_large_template_expansion	passed	1.2312	1.1994 (2d42cdb)	1.1989
test_many_experiments	passed	24.0619	23.2386 (2d42cdb)	23.6468
test_many_objects_defaults	passed	13.6165	12.1959 (2d42cdb)	12.5012
test_matrix_filter_perf	passed	1.1793	1.0590 (2d42cdb)	1.0757

[linsword13]

/gemini review

[gemini-code-assist]

Code Review

This pull request configures a private N2D worker pool in the southamerica-west1 region for Ramble unit tests and updates the Cloud Build triggers to use this pool along with a repository connection config. Feedback on these changes highlights that the repository connection strings hardcode the owner and repository name, which bypasses existing configuration variables. Additionally, pulling builder images across regions from us-central1 to southamerica-west1 may introduce latency and egress costs, and defaulting the _WORKER_POOL substitution to an empty string could trigger Cloud Build validation failures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.04%. Comparing base (279e771) to head (ab11a70).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1750   +/-   ##
========================================
  Coverage    94.04%   94.04%           
========================================
  Files          370      370           
  Lines        37331    37331           
========================================
  Hits         35109    35109           
  Misses        2222     2222           

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[rfbgo]

@dcoapp recheck