[Code Quality] Extract validation logic from `main.py` into `validation.py` module

[github-actions]

Stage 3 of the modularization plan outlined in Daily Backlog Burner discussion #493. The plan sequences extractions as config.py (#473) → api_client.py (#516) → validation.py → cache.py. This issue tracks stage 3.

main.py is ~3,200 lines. Grouping all validation logic into a dedicated module makes it easier to test validators in isolation and prevents them from becoming entangled with HTTP or config concerns.

Suggested Changes

Extract the following functions from main.py into a new validation.py:

Function	Line (approx.)	Purpose
get_validated_input()	~495	Prompt + validate user input
_validate_config()	~608	Validate loaded config dict
validate_hostname()	~1008	Validate DNS hostname format
validate_folder_url()	~1051	Validate block-list URL
is_valid_profile_id_format()	~1093	Check profile ID pattern
validate_profile_id()	~1110	Full profile ID validation
validate_folder_id()	~1129	Full folder ID validation
is_valid_rule()	~1149	Validate a DNS rule entry
is_valid_folder_name()	~1168	Validate folder name
validate_folder_data()	~1202	Validate fetched folder JSON

Implementation steps:

1. Create validation.py with all listed functions
2. Update main.py to from validation import ...
3. Update any test mocks that patch main.validate_hostname etc. to point to the new module
4. Run uv run pytest tests/ -v to confirm no regressions

Files Affected

• main.py — remove extracted functions, add import
• validation.py — new file
• tests/test_folder_validation.py, tests/test_fix_broken_validation.py — update patch() targets if needed

Success Criteria

• ✅ All validation functions moved to validation.py
• ✅ main.py imports from validation.py
• ✅ All 30 test modules pass: uv run pytest tests/ -v
• ✅ main.py line count reduced by ~200 lines

Source

Extracted from Daily Backlog Burner discussion #493 — modularization roadmap stage 3.

Priority

Medium — Builds directly on config (#473) and api_client (#516) extraction. Improves testability of individual validators and keeps main.py focused on orchestration logic.

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• expires on Mar 4, 2026, 1:40 PM UTC

[linear]

ABH-483