Add comprehensive tests for tree size validation

[0x687931]

Summary

Implements comprehensive test suite for fetch_tree() tree size limit validation as specified in Fix #2. Tests validate that the OTA client properly rejects trees that exceed configured size or file count limits.

Test Coverage

Core Test Cases (as required):

1. test_fetch_tree_normal_size: 100 files under 200-file limit → succeeds
2. test_fetch_tree_too_many_files: 500 files over 200-file limit → raises OTAError with helpful message
3. test_fetch_tree_size_limit_via_header: Large Content-Length header → raises OTAError before parsing
4. test_fetch_tree_custom_limits: Custom config limits (50 files) → properly enforced
5. test_fetch_tree_helpful_error_message: Error messages include actual count, limit, and guidance

Additional Edge Cases:

• test_fetch_tree_custom_limits_passes: Custom limits allow valid trees through
• test_fetch_tree_default_limits: Default limits work when config doesn't specify
• test_fetch_tree_empty_tree: Empty trees handled correctly
• test_fetch_tree_at_exact_limit: Trees at exact limit succeed
• test_fetch_tree_one_over_limit: Trees one file over limit fail

Implementation Notes

• Follows established test patterns from test_path_filtering.py and test_verify.py
• Uses monkeypatch for dependency isolation (no real network calls)
• Mock response objects (Resp, RespWithContentLength) simulate GitHub API responses
• Validates both error detection and helpful error messaging
• Tests both file count limits (max_tree_files) and byte size limits (max_tree_bytes)

Test File

• Location: /Users/am/Documents/GitHub/ota/tests/test_tree_limits.py
• Lines: 370
• Test count: 10 comprehensive test functions

Dependencies

These tests expect the fetch_tree() method to be updated with validation logic that:

1. Checks Content-Length header against max_tree_bytes config (if set)
2. Parses tree response and validates file count against max_tree_files config
3. Raises OTAError with helpful messages including actual values, limits, and guidance

Ready to run with pytest tests/test_tree_limits.py once validation is implemented.

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