feat(core): add explicit config validation on load

[errfld]

What changed

• added a dedicated SmeeConfig::validate() pass and invoke it during config load
• reject empty hook lists and empty/whitespace commands with explicit ValidationError variants that include hook name and entry index
• reject unknown fields in HookDefinition via #[serde(deny_unknown_fields)]
• expanded config integration coverage for whitespace commands and unknown hook entry fields
• added a CLI integration test verifying install reports config validation failures
• removed a stray assertion from installer_integration.rs that prevented workspace tests from compiling

Validation

• cargo test --workspace

Closes git-smee-4q5.

Summary by CodeRabbit

• New Features

  • Configuration validation now enforces non-empty hook entries and commands, reporting detailed errors that identify the specific hook and entry with issues.
  • Unknown configuration fields are now rejected during parsing.

• Tests

  • Added integration tests covering validation error reporting for invalid configurations.

[coderabbitai]

📝 Walkthrough

Walkthrough

Configuration validation is introduced with a new ValidationError enum supporting EmptyHookEntries and EmptyCommand variants. The SmeeConfig::validate() method enforces non-empty hook entries and commands after parsing. The from_toml() method now invokes validation, and HookDefinition gains deny_unknown_fields to reject unexpected fields in TOML.

Changes

Cohort / File(s)	Summary
Config Validation System crates/git-smee-core/src/config.rs	Introduces ValidationError enum with EmptyHookEntries and EmptyCommand variants. Adds SmeeConfig::validate() method to enforce non-empty entries and commands. Updates from_toml() to call validation after parsing. Adds #[serde(deny_unknown_fields)] to HookDefinition. Integrates ValidationError into main Error enum.
CLI Integration Tests crates/git-smee-cli/tests/cli_integration.rs	Adds test given_invalid_config_when_install_then_validation_error_is_reported() to verify install command reports validation errors with correct hook_name and entry_index in stderr.
Config Unit Tests crates/git-smee-core/tests/config_integration.rs	Adds tests verifying whitespace-only commands trigger ValidationError::EmptyCommand and unknown fields in hook definitions produce parsing errors with descriptive messages.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Config validation hops along
With empty commands ringing strong,
ValidationErrors catch the flaw,
Unknown fields denied by serde law!
Safety checks from the core so bright, 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat(core): add explicit config validation on load' accurately and concisely describes the main change: adding explicit validation to the config loading process.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch git-smee-4q5/config-validation-errors

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