Add durability contract tests and harden HTTP fd handling

[kacy]

What changed

• add durability-focused test-contract coverage for S3 object persistence and multipart staging cleanup
• wire the new restart and durability contract module into the contract test target
• make the contract target depend on the installed binary build step
• harden HTTP connection timeout setup so raw contract harnesses using non-socket file descriptors do not panic

Why

The next confidence step was persistence-oriented contract coverage. While adding that coverage, the raw HTTP contract path exposed a reliability bug in the connection runtime when timeout setup ran against a non-socket fd.

Impact

• improves release confidence around durable S3 writes and multipart cleanup behavior
• keeps the contract suite deterministic and stable
• removes a panic in contract-style in-process HTTP handling

Validation

• zig build test-contract -Doptimize=ReleaseSafe
  • All 21 tests passed.