Improve error details and handle proxy connection errors

[kmcginnes]

Description

Incremental improvements to error handling and display, building on #1644:

• Introduce ServerConnectionError for when fetch fails to reach the server, capturing the request URL and original cause for diagnostics
• Detect cross-origin mismatches between the proxy server URL and the browser origin, showing a targeted CORS error message
• Consolidate the private extractMessageFromData in createDisplayError.ts into the shared extractErrorMessage utility and move it from connector/ to utils/
• Simplify decodeErrorSafely to return raw text for non-JSON responses instead of wrapping in { message }, since extractErrorMessage handles plain strings directly
• Add statusText lookup to NetworkError for human-readable status names in error details
• Widen ErrorDetails.message to string | undefined and conditionally hide the message field in the error details dialog when undefined
• Add a comment explaining why serializeCause sets name explicitly (it lives on the prototype, not as an own property)
• Wrap all TypeError exceptions from fetch() as ServerConnectionError for cross-browser compatibility (Chrome, Firefox, Safari use different messages for network failures)
• Tighten hasOriginMismatch parameter type from string | undefined to string
• Add comprehensive test suites for fetchDatabaseRequest, NetworkError, ServerConnectionError, createDisplayError, and createErrorDetails

Validation

• pnpm checks passes (lint, format, types)
• pnpm test passes with no failures

Related Issues

• Resolves Connection timeout results in TypeError #1503
• Part of Improve error details with richer diagnostic information #1644

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.88%. Comparing base (c5affc5) to head (8362333).
⚠️ Report is 122 commits behind head on main.

Files with missing lines	Patch %	Lines
...kages/graph-explorer/src/components/PanelError.tsx	75.00%	1 Missing ⚠️
...ges/graph-explorer/src/utils/createDisplayError.ts	94.73%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1650       +/-   ##
===========================================
+ Coverage   47.81%   63.88%   +16.06%     
===========================================
  Files         382      396       +14     
  Lines        8525     9247      +722     
  Branches     3159     3521      +362     
===========================================
+ Hits         4076     5907     +1831     
+ Misses       3070     2310      -760     
+ Partials     1379     1030      -349     

Flag	Coverage Δ
unittests	63.88% <95.12%> (+16.06%)	⬆️

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