Adds missing onerror callback invocations before every createJsonErrorResponse
call in WebStandardStreamableHTTPServerTransport. This ensures that transport
errors are no longer silently swallowed and can be observed via the onerror
callback.

Changes:
- Add this.onerror?.() calls to 15 locations in streamableHttp.ts
- Add 10 test cases to verify onerror is called for various error conditions

Fixes modelcontextprotocol#1395

… errors

- Add onerror for writeSSEEvent catch (surfaces actual write failure)
- Add onerror for handleUnsupportedRequest (405 method not allowed)
- Surface the caught SyntaxError/ZodError in JSON and JSON-RPC parse catches
  instead of constructing a generic 'Parse error' string — more useful for
  debugging
- Update tests to assert on error types (SyntaxError, ZodError)

…r messages

- Remove redundant onerror call in replayEvents send callback (writeSSEEvent already reports the actual error)
- Add trailing period to handleUnsupportedRequest onerror message to match response body
- Include supported versions list in validateProtocolVersion onerror message to match response body