Make test-product retention configurable

[ianrahman]

What changed

• add testProductsMaxCount and testProductsMaxAgeDays project configuration, with XCODEBUILDMCP_TEST_PRODUCTS_MAX_COUNT and XCODEBUILDMCP_TEST_PRODUCTS_MAX_AGE_DAYS environment overrides
• pass dedicated test-product retention limits through the workspace filesystem lifecycle
• lower managed .xctestproducts retention from 100 bundles / 3 days to 3 bundles / 1 day
• preserve the existing safety boundary: only XcodeBuildMCP-managed paths are pruned; caller-provided prepared-test paths are untouched
• cover defaults, configuration precedence, schema validation, and lifecycle pruning

Why

Source-based test runs stage a full .xctestproducts bundle under the workspace storage directory. For multi-gigabyte test products, the previous 100-bundle default permits hundreds of gigabytes of retained data before count-based cleanup applies.

Validation

• TypeScript syntax checks for all changed source and test files
• direct retention-pruning smoke test
• configuration default and precedence smoke test
• workspace lifecycle integration smoke test (3 scanned, 2 deleted, newest retained)
• git diff --check

Fixes #524.

AI assistance: Codex helped trace the retention lifecycle, implement the configuration surface, and run the available validation. The resulting diff and test evidence were reviewed before submission. (Though tbh I'm not a Typescript guy and this could use more experienced review for that reason.)