feat(rfq): expose RFQ error IDs

[AnanthVivekanand]

Summary

• Parse RFQ error_id payloads.
• Expose error_id on RFQ rejection errors.
• Recognize INVALID_SIGNATURE and INTERNAL_ERROR.

Validation

• make check
• RFQ integration tests require credentials and were skipped locally.

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Note

Low Risk
Small additive API on RFQ error types and stricter validation only when error_id is present; no auth or trading path changes.

Overview
RFQ websocket RFQ_ERROR responses can include an error_id for support and correlation. The quoter session now reads that field (optional string), rejects malformed non-string values with UnexpectedResponseError, and attaches error_id on RfqQuoteRejectedError, RfqCancelQuoteRejectedError, and RfqConfirmationRejectedError.

RfqErrorCode adds INVALID_SIGNATURE and INTERNAL_ERROR so those server codes parse without unknown-code failures. Integration coverage asserts error_id on quote rejection.

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