fix(server): surface stateless transport reuse errors

[Genmin]

Summary

• return a JSON-RPC error response when a stateless Streamable HTTP transport instance is reused
• invoke transport.onerror for that reuse error instead of letting the exception become an opaque Hono 500
• document the v1.x stateless lifecycle expectation on both the Node and WebStandard transports

Context

Addresses the visibility and diagnostics part of #1994. The existing v1.x code intentionally guards against reusing a stateless transport instance; this PR keeps that lifecycle rule, but makes the failure observable and parseable. That turns the current empty/text 500 into the SDK's normal JSON-RPC error shape and calls onerror with the underlying error message.

Validation

• npx vitest run test/server/streamableHttp.test.ts -t "should return a JSON error when a stateless transport is reused with a pre-parsed body"
• npx vitest run test/server/streamableHttp.test.ts -t "should call onerror and return JSON when stateless transport is reused"
• npx vitest run test/server/streamableHttp.test.ts
• npm run typecheck -- --pretty false
• npx prettier --check src/server/streamableHttp.ts src/server/webStandardStreamableHttp.ts test/server/streamableHttp.test.ts .changeset/stateless-transport-reuse-error.md
• git diff --check

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🦋 Changeset detected

Latest commit: 0c3595b

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commit: 0c3595b