fix(gui): extract nested messages from structured API errors

[hopstreax]

Description

Fixes #12945.

This PR improves structured API error parsing in the GUI by extracting nested error.message values instead of displaying the entire stringified error object.

Previously, when providers returned structured error payloads (such as Gemini's HTTP 429 quota-exceeded response), the View error output section displayed raw JSON (for example, {"message":"...","status":"RESOURCE_EXHAUSTED"}) rather than the provider's human-readable error message.

This change updates the error parser to:

• Extract and display the nested error.message when present.
• Preserve the existing behavior for primitive error values, malformed JSON, top-level message fields, and structured error objects without a message field.
• Keep the existing HTTP status code extraction and provider-specific error handling unchanged.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Since reproducing the original issue requires exceeding the Gemini API quota, I wasn't able to capture a runtime screenshot.

Instead, I verified the fix by simulating the original error payload reported in the issue.

Before

{"message":"You exceeded your current quota for Gemini API in your project. Please check your plan and billing details.","status":"RESOURCE_EXHAUSTED"}

After

You exceeded your current quota for Gemini API in your project. Please check your plan and billing details.

Tests

• Updated the existing tests for structured nested error parsing.
• Added regression tests covering:
  • Structured error objects with a nested message
  • Structured error objects without a message
  • Primitive string errors
  • Top-level message fields
  • Malformed JSON
• Simulated the original Gemini quota-exceeded payload from Issue Error: Gemini 2.5 Pro - Unknown error #12945 and verified that:
  • The parser extracts the human-readable error message.
  • HTTP status code extraction (429) remains unchanged.
• Verified that all errorAnalysis tests pass successfully.

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[hopstreax]

I have read the CLA Document and I hereby sign the CLA