feat: make --config-file take precedence over environment variables

[joshrotenberg]

Summary

Fixes #436 - When --config-file is explicitly specified, environment variables are now ignored to provide true configuration isolation.

Problem

Previously, environment variables always took precedence over config files, even when --config-file was explicitly specified. This made it impossible to achieve true configuration isolation for testing and violated the principle of "explicit wins" in CLI design.

Old behavior (problematic):

Priority: Env vars > --config-file > Default config

Solution

New behavior:

Priority: --config-file (explicit) > Env vars > Default config

When --config-file is explicitly provided, environment variables are completely ignored for credential resolution.

Changes

1. ConnectionManager: Tracks whether config_path was explicitly provided
2. Credential Resolution:
   • create_cloud_client() and create_enterprise_client() skip env var checks when config_path is Some()
   • Added debug logging showing when env vars are ignored
3. API Command: Fixed bug where ApiCommandParams wasn't preserving config_path, causing it to be lost
4. Tests: Added 2 new integration tests to verify the behavior

Testing

New Tests

• test_config_file_overrides_env_vars: Verifies env vars are ignored when --config-file is specified
• test_env_vars_work_without_config_file: Verifies env vars still work without --config-file

Test Results

redisctl tests: 132 passed (31 unit + 24 basic CLI + 29 mocked + 15 profile + 2 output + 31 other)

Benefits

1. Testing Isolation: Tests can use --config-file without worrying about env vars
2. Principle of Least Surprise: Explicit flags override implicit environment
3. CLI Best Practice: Follows patterns from kubectl --kubeconfig, git --config, etc.
4. Backward Compatible: Only affects behavior when --config-file is explicitly used

Example

Before (env vars override config file):

export REDIS_CLOUD_API_KEY=production-key
redisctl --config-file test.toml cloud subscription list
# Uses production-key (WRONG!)

After (config file wins when explicit):

export REDIS_CLOUD_API_KEY=production-key  
redisctl --config-file test.toml cloud subscription list
# Uses test.toml credentials (CORRECT!)

Without --config-file (env vars still work):

export REDIS_CLOUD_API_KEY=my-key
redisctl cloud subscription list
# Uses my-key from env (works as before)

Closes #436