test(integration): [#1419] use explicit config path fixtures

[josecelano]

Summary

Migrates the shared main-level integration fixture from process-wide configuration environment mutation to the explicit configuration-file startup API added by #2151.

Changes

• TrackerApplicationFixture now passes its existing isolated workspace TOML file to app::start_with_explicit_config_toml_path.
• Removes the fixture's global environment lock, environment restoration guard, and unsafe std::env mutation.
• Updates tests/AGENTS.md and issue Allow multiple integration tests at the main app level #1419 to distinguish the resolved configuration-injection constraint from the still-deferred process-global lifecycle and shutdown constraints.

Validation

• All eight current main-level integration targets passed together.
• Focused tracker Clippy and cargo fmt --check passed.
• Markdown, spelling, local-link, and whitespace checks passed.
• Required pre-commit gate passed.

Related to #1419

This PR does not close #1419. Cooperative server shutdown and the post-#1488 fixture review remain tracked there.

[Copilot]

🟢 Approval recommended

The functional change is low-risk and aligns with #2151’s explicit-config startup flow; the only noted follow-up is a minor issue-spec metadata formatting tweak.

Pull request overview

This PR updates the main-level integration-test fixture to start the tracker using the explicit config TOML path entry point added in #2151, eliminating process-wide configuration environment mutation and its associated synchronization guard.

Changes:

• Switched TrackerApplicationFixture startup to app::start_with_explicit_config_toml_path(...) using the workspace-local config file.
• Removed the global environment lock and unsafe std::env mutation/restoration guard from the shared integration workspace helper.
• Updated test documentation and issue #1419 notes to reflect that base-source configuration injection via environment is no longer required for in-process fixtures.

File summaries

File	Description
tests/common/workspace.rs	Removes process-global env mutation and starts the app using an explicit config TOML path.
tests/AGENTS.md	Updates test guidance to reflect explicit-path startup for in-process fixtures.
docs/issues/open/1419-allow-multiple-integration-tests-at-main-app-level/ISSUE.md	Updates issue narrative and acceptance-criteria notes to reflect the fixture migration away from env-based base-source injection.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.97%. Comparing base (afd995f) to head (708cb81).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2189      +/-   ##
===========================================
- Coverage    84.97%   84.97%   -0.01%     
===========================================
  Files          351      351              
  Lines        30211    30208       -3     
  Branches     30211    30208       -3     
===========================================
- Hits         25672    25668       -4     
  Misses        4168     4168              
- Partials       371      372       +1     

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[josecelano]

ACK 708cb81