feat(internal/sidekick/rust): initialize bidi streaming RPCs eagerly

[suzmue]

Initialize bidirectional streaming RPCs eagerly on send instead of deferring transport dialing until the first message is written to the channel in a background task.

Stream builders now support field setters and with_request to configure the initial request message, and send initiates the stream immediately, surfacing connection errors to the caller.

This change is intended to be the minimal change needed to move the initialization logic and does not address other TODOs remaining in the transport implementation. This includes configuration for the channel values, which may mean we will not use RequestBuilder directly in the final implementation.

For #6835

[gemini-code-assist]

Code Review

This pull request refactors the Rust template code for bidirectional streaming RPCs. It renames the builder's build method to send, which now accepts an initial request and returns a Result. The stub, tracing, and transport layers are updated to propagate this request and return a Result. In the transport layer, the connection setup is now awaited directly, allowing stream initiation errors to be returned immediately to the caller. I have no feedback to provide as there are no review comments.

[coryan]

In Swift we started writing unit tests to verify the mustache template output matches expectations.... For example:

librarian/internal/sidekick/swift/generate_enum_swift_test.go

Lines 90 to 103 in 5a7775a

	got := extractBlock(t, string(contentsB), "/// Initialize from an integer value.", "\n }")
	want := `/// Initialize from an integer value.
	///
	/// If the value is unknown, this initializes to ` + "[`unknownIntValue`](doc:Kind/unknownIntValue(_:))." + `
	public init(intValue: Int) {
	switch intValue {
	case 0: self = .test
	case 1: self = .otherTest
	default: self = .unknownIntValue(intValue)
	}
	}`
	if diff := cmp.Diff(want, got); diff != "" {
	t.Errorf("mismatch (-want +got):\n%s", diff)
	}

I think these are less annoying than golden file tests, and will avoid somebody accidentally changing the template to return the wrong thing.

The downside is that they trigger errors if you change spaces or indentation in the mustache templates.

Consider starting a few such tests with this PR.