test: close PSC and Transit Gateway coverage gaps

[joshrotenberg]

Summary

Claim issue #146 and bring Private Service Connect and Transit Gateway request/response coverage in line with the neighboring connectivity modules.

Closes #146.

Changes

• Add dedicated wiremock suites for PSC standalone and Active-Active routes.
• Cover PSC create/get/update/delete plus endpoint listing, creation scripts, deletion scripts, and endpoint deletion.
• Add dedicated wiremock suites for Transit Gateway standalone and Active-Active routes.
• Cover gateway listing, invitation listing/accept/reject, attachment create/update/delete, and the legacy compatibility alias.
• Assert authentication headers, exact paths, request wire keys, response deserialization, task responses, and error mappings.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• Focused PSC/TGW suites: 8 passed
• cargo test --workspace
• cargo llvm-cov --all-features --summary-only

Coverage results:

• connectivity/psc.rs: 29.29% → 100% lines, 35.14% → 100% functions, 35.04% → 100% regions
• connectivity/transit_gateway.rs: 28.35% → 100% lines, 31.03% → 100% functions, 32.65% → 100% regions
• Total crate line coverage: 68.31% → 76.37%

The final diff was reviewed and contains two dedicated test files with no production behavior change. The existing unified-handler test continues to cover the legacy list_tgw_attachments alias.

Stack

This draft temporarily targets #158's branch so the issue is visibly claimed while #158 runs CI. It will be rebased and retargeted to main after #158 merges.