mcp: do not cache resources/read results of multi round-trip retries

[vbcherepanov]

ClientSession.ReadResource cached every successful result under its URI and looked every request up by URI alone. The multi round-trip middleware runs inside handleSend, so the result of a retry carrying inputResponses and requestState was cached and served on the next plain read, which bypassed the elicitation that gated the resource. With the middleware disabled, a caller's manual retry was answered from the cache with the interim input_required result of the first read and never reached the server.

This change keeps multi round-trip traffic out of the cache in both directions:

• A request that carries InputResponses or RequestState is neither served from the cache nor stored.
• The client middleware marks a ReadResourceResult it obtained by retrying, through a small unexported retriedResult interface, and ReadResource does not store such a result.
• An input_required result is not stored.

Results that needed no input are cached as before. There is no change to the exported API or to the wire format; the marker is an unexported field.

The specification says results of retries "MUST NOT be cached, as they depend on inputs that are not part of the cache key", that a cached response must not be served "for a request whose method or parameters differ from the request that produced it", and that interim results "are not cacheable" (2026-07-28, Caching).

Fixes #1369

Tested with go test ./... -race and the new tests at -race -count=50. TestMultiRoundTrip_ReadResource_RetryResultNotCached covers the automatic retry and includes a positive control for a plain cached resource. TestMultiRoundTrip_ReadResource_ManualRetryBypassesCache covers the manual retry loop. Both fail on main and pass with this change.

[guglielmo-san]

Suggested change

	if reqMap == nil && !mrtrResult.NeedsInput() {
	if r, ok := res.(retriedResult); ok && retries > 1 {
	r.markRetried()
	}
	return res, nil
	}

[vbcherepanov]

Applied in 8aa288c. I kept the !res.isNil() check from the earlier version: inputRequests() is nil-safe, so a typed nil *ReadResourceResult can reach this branch, and markRetried() would dereference it. go test ./mcp/ passes, and the multi round-trip tests pass with -race.

[guglielmo-san]

@vbcherepanov thank you for the contribution!