Invert the unary path onto an async spine

[eseay]

Stacked on #432 (targets eddie/with-deadline-helper) — this is what gives withDeadline its first caller.

What this does

ProtocolClient's async unary() was a wrapper around the callback version via UnaryAsyncWrapper. This flips that: async unary() is now the real implementation, and the callback version is a thin Task wrapper around it.

To do that without breaking anyone, Interceptor, UnaryInterceptor, and HTTPClientInterface each gain async versions of their methods as protocol requirements with default implementations that call through to the existing callback versions. No new public types, no adapters — every existing conformer still compiles and still gets called.

Key decision

Defaults only point one direction: async falls back to callback, never the reverse. HTTPClientInterface's callback unary() deliberately keeps no default, because giving it one would let both sides default to each other — a type implementing neither would loop forever with no compile error. That case is covered by a test that would hang, not fail, if this were ever violated.

Behavior changes

• Canceling mid-request used to make an async caller hang forever (the completion was just never called). It now returns a .canceled response. The callback path keeps today's behavior for now.
• A response synthesized after losing the deadline race no longer carries over headers/trailers from a canceled response. Both built-in HTTP clients already report those as empty in that case, so this is a no-op in practice.
• Completion callbacks now run on the general concurrent executor instead of URLSession's serial delegate queue — worth knowing if anything relied on serial delivery.

Scope

Unary only. Streaming still uses the old callback chain and TimeoutTimer for now.

Testing

swift test (101/101), make testconformance (966/966 URLSession + 1681/1681 NIO), SwiftLint clean, iOS/macOS builds green. TimeoutTests.swift passes unmodified, which is the strongest signal the deadline bridge behaves correctly.