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 test coverage for environment variable-based configuration, including validation of individual variables, override behavior with YAML configuration, and error handling for invalid values.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Review profile: CHILL

Plan: Enterprise

Run ID: 29007d54-6c49-41f9-beb9-ed377cb98a62

📥 Commits

Reviewing files that changed from the base of the PR and between c9e27b6 and b9ed127.

📒 Files selected for processing (1)

• fact/src/config/tests.rs

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📝 Walkthrough

Walkthrough

This PR adds concurrency-safe test infrastructure and three comprehensive test cases validating that environment variable bindings (FACT_INODES_MAX, etc.) correctly populate FactConfig fields, override YAML configuration, and properly error on invalid inputs.

Changes

Environment Variable Test Coverage

Layer / File(s)	Summary
Thread-safe test harness and helpers fact/src/config/tests.rs	Imports Mutex, MutexGuard, and error/display traits; defines global ENV_MUTEX and EnvVarGuard RAII type that removes environment variables on drop while holding the lock; provides EnvVar wrapper with set() method and Display implementation; implements with_env_var() helper that sets an env var and parses CLI into FactConfig under mutex protection to prevent test interference.
Environment variable binding tests fact/src/config/tests.rs	env_vars test iterates over FACT_* environment variables and asserts the parsed config matches expected field values. env_vars_override_yaml test verifies that env vars override corresponding YAML-sourced config values. env_vars_invalid_values test validates that invalid env var inputs produce parse/validation errors with expected source strings.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 A mutex guard hops through the test fair,
where env vars dance without a care—
locked, set, parsed, then cleaned with grace,
no race conditions in this space! 🔒✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% 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 PR title accurately and concisely summarizes the main change: adding unit tests for environment variable bindings.
Description check	✅ Passed	The description covers the key changes, testing approach, and checklist items; however, Testing Performed section is incomplete with only 'Run the tests manually.'
Linked Issues check	✅ Passed	The PR successfully implements all coding requirements from issue #730: comprehensive env-var tests (individual vars, precedence over YAML, invalid values), mutex-based serialization for race condition mitigation, and proper cleanup.
Out of Scope Changes check	✅ Passed	All changes in the PR are focused on adding unit tests for environment variable bindings as specified in issue #730; no out-of-scope modifications 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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