client: suppress duplicate abort errors during StreamableHTTP close

[windro-xdd]

Summary

• add a close-state guard in StreamableHTTPClientTransport to suppress abort-driven onerror emissions while shutdown is in progress
• keep normal error behavior unchanged when not closing
• route internal error reporting through a single _emitError helper to avoid duplicate callback paths
• add regression tests for client.close() abort behavior

Why

Issue #868 reports duplicate onerror callbacks when closing a Streamable HTTP client. During shutdown, in-flight requests can reject with AbortError, and those errors should not be surfaced as user-facing failures after an intentional close.

Validation

• pnpm --filter @modelcontextprotocol/client test -- test/client/streamableHttp.test.ts
• pnpm --filter @modelcontextprotocol/client typecheck
• pnpm --filter @modelcontextprotocol/client lint

Closes #868

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