chore: code cleanup - narrow visibility, extract constant, fix httpmock 0.8 API, fix workspace-hack

[loonghao]

Code Cleanup (Cycle #4)

Automated code cleanup pass for the fpt-cli project.

Changes

1. Extract TOKEN_CACHE_POISONED constant (transport.rs)

• Extracted duplicate "token cache is poisoned" string literal into a named constant
• Used in both read_token_cache and write_token_cache operations

2. Narrow plan_entity_* visibility (transport.rs)

• Changed plan_entity_create, plan_entity_update, plan_entity_delete, plan_entity_revive from pub to pub(crate)
• These functions are only used within the fpt-domain crate and are not re-exported in lib.rs

3. Fix httpmock 0.8 API migration (rest_transport_tests.rs)

• body_contains() → body_includes() (3 occurrences)
• .hits() → .calls() (110 occurrences)
• Pre-existing test compilation failure now resolved

4. Fix workspace-hack duplicate dependency (workspace-hack/Cargo.toml)

• Merged two windows-sys v0.61 entries with different feature sets into a single unified entry
• Resolved pre-existing cargo check / cargo clippy build failure

Verification

• ✅ cargo fmt --all — formatted
• ✅ cargo clippy --workspace — zero warnings
• ✅ cargo test --workspace — all tests pass (97 passed, 2 doc-tests ignored)