Conform HTTPMethod to Sendable

[radimvaculik]

Motivation

FTAPIKit 2.0 advertises full Swift 6 concurrency safety, and the async API (URLServer.call(endpoint:)) hands the endpoint to a @concurrent context. Endpoint.method is a stored property on most endpoints, so as soon as a consumer marks its endpoint types Sendable — which the async API effectively requires — the compiler reports:

warning: stored property 'method' of 'Sendable'-conforming struct 'FooEndpoint'
has non-Sendable type 'HTTPMethod'; this is an error in the Swift 6 language mode

The only workaround on the consumer side is a retroactive conformance:

extension HTTPMethod: @retroactive @unchecked Sendable {}

which is awkward and would break the moment FTAPIKit declares the conformance itself.

Change

HTTPMethod is a plain String-backed enum with no associated values, so it is trivially Sendable. This just states it.

Adding a conformance is additive — no existing code breaks.

Not included

Two related gaps I deliberately left out because they need a design decision rather than a one-liner:

• MultipartBodyPart stores an InputStream, so it is not safely Sendable as-is. Consumers of MultipartEndpoint currently have to fall back to @unchecked Sendable on their endpoint types.
• Endpoint: Sendable would be the complete fix, since the async API already sends endpoints across isolation domains, but it is source-breaking for existing conformances.

Happy to follow up on either if you have a preferred direction.

Verification

swift build passes. The test suite could not be run to completion here — ErrorHandlingTests and EndpointTypeTests hit httpbin.org over the network and time out in this environment; the failures are unrelated to this change.