test: add unit tests for environment variable bindings

[Molter73]

Description

Adds coverage for all FACT_* env-var bindings in FactCli:

• env_vars: each env var populates the correct FactConfig field
• env_vars_override_yaml: env vars take precedence over YAML config
• env_vars_invalid_values: invalid values produce ValueValidation errors

Introduces EnvVar and EnvVarGuard helper types that together acquire ENV_MUTEX, set the variable, and guarantee removal on drop — even if the test panics. std::env::set_var is not safe in multi-threaded programs; the mutex serialises all env-var mutations within the suite as the least invasive available mitigation.

Closes #730

Assisted-by: Claude Sonnet 4.6

Checklist

• Patch has a change log entry OR does not need one.
• Investigated and inspected CI test results
• Updated documentation accordingly

Automated testing

• Added unit tests
• Added integration tests
• Added regression tests

If any of these don't apply, please comment below.

Testing Performed

Run the tests manually.

Summary by CodeRabbit

• Tests
  • Added comprehensive tests for environment-variable configuration with concurrency-safe isolation and automatic cleanup of temporary env vars.
  • Validates parsing of multiple variables (paths, numbers, booleans, URL, socket), verifies env-vars override YAML settings, and checks clear error messages for invalid values.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between b9ed127 and cedeabe.

📒 Files selected for processing (1)

• fact/src/config/tests.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• fact/src/config/tests.rs

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📝 Walkthrough

Walkthrough

This PR adds concurrency-safe test infrastructure and three tests that verify FACT_* environment variables populate FactConfig fields, override YAML configuration when applied, and produce clap parse/validation errors for invalid values.

Changes

Environment Variable Test Coverage

Layer / File(s)	Summary
Thread-safe test harness and helpers fact/src/config/tests.rs	Imports Display and Mutex/MutexGuard; defines global ENV_MUTEX, EnvVarGuard RAII cleanup, EnvVar wrapper with a locked set() method and Display; adds with_env_var() helper that sets an env var and parses CLI into FactConfig while holding the mutex.
Environment variable binding tests fact/src/config/tests.rs	Adds env_vars which iterates FACT_* env vars and asserts parsed FactConfig fields; env_vars_override_yaml which parses YAML, applies an env var via config.update(...), and asserts the env var overrides YAML; env_vars_invalid_values which asserts invalid env var values produce clap::Error messages with expected first-line diagnostics.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 I hop in tests with mutex tight,
set an env var, parse it right,
then gently drop and wipe the trace —
no races in this tranquil place. 🥕

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main change: adding unit tests for environment variable bindings in the codebase.
Description check	✅ Passed	The PR description covers the key changes, includes all required checklist items (with reasoning for unchecked items), and documents testing approach. Minor: 'Updated documentation accordingly' is unchecked but appropriate since tests don't require doc changes.
Linked Issues check	✅ Passed	The PR fully implements all requirements from issue #730: comprehensive unit tests for env-var bindings [#730], precedence over YAML config [#730], invalid-value error handling [#730], mutex-based serialization to prevent race conditions [#730], and cleanup via EnvVarGuard RAII [#730].
Out of Scope Changes check	✅ Passed	All changes are directly aligned with issue #730 requirements: test harness (EnvVar/EnvVarGuard/ENV_MUTEX), env-var binding tests, YAML precedence tests, and invalid-value tests. No extraneous changes detected.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch mauro/chore/add-env-var-unit-tests

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[Molter73]

/retest fact-on-push