test: add unit tests for environment variable bindings (e.g. FACT_INODES_MAX)

[coderabbitai]

Summary

Add unit tests that verify environment variable bindings correctly populate the corresponding configuration values.

Currently there are no tests covering env-var bindings for CLI arguments such as FACT_INODES_MAX, FACT_INODE_MAX (old name), and other env-vars mapped in FactCli. These were deferred because std::env::set_var is process-global and inherently racy when tests run in parallel under the same process.

Motivation

The rename of FACT_INODE_MAX → FACT_INODES_MAX in #729 highlighted the lack of test coverage for env-var bindings. A regression here (wrong env-var name in the #[arg(env = ...)] attribute) would not be caught by existing tests.

Suggested approach

• Serialize env-var tests using a shared Mutex (or the serial_test crate) so they don't interfere with parallel tests.
• Alternatively, scope tests to run with RUST_TEST_THREADS=1 or via a dedicated test binary target.
• At minimum, add coverage for FACT_INODES_MAX populating bpf.inodes_max, and consider covering other env-var bindings in FactCli.
• Ensure each test cleans up the env var (e.g., via std::env::remove_var) to prevent cross-test pollution.

References

• PR: cleanup: improve error message when inode map is full #729
• Comment: cleanup: improve error message when inode map is full #729 (comment)

/cc @Molter73