Swap route error message fix

[neavra]

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of plugin validation failures: validation errors now return a clear, user-facing message and a 400 response so users get actionable feedback.
  • Other plugin errors continue to surface as before (no change to non-validation error behavior).

[coderabbitai]

Walkthrough

Adds a ValidationError type for user-facing plugin validation messages, returns HTTP 400 from the plugin server when such errors occur, and updates the API layer to detect and extract validation messages from plugin HTTP 400 responses.

Changes

Cohort / File(s)	Summary
Plugin Error Type plugin/errors.go	Adds ValidationError (fields Err error, Message string), implements Error() and Unwrap(), and adds NewValidationError constructor.
Validation handling (server + API) plugin/server/server.go, internal/api/plugin.go	Server: detect *plugin.ValidationError and respond with HTTP 400 and the validation message. API: import github.com/vultisig/verifier/plugin/libhttp and parse plugin HTTP 400 (libhttp.HTTPError) to extract JSON validation messages for API responses.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title refers to a 'swap route error message fix' but the changeset adds generalized plugin validation error handling with new error types and updates multiple files beyond swap routes.	Update the title to reflect the broader scope, such as 'Add plugin validation error handling' or 'Improve plugin error message handling for HTTP 400 responses'.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch swap-route-fix-error-message

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@internal/api/plugin.go`:
- Around line 703-715: The handler treats plugin validation errors as HTTP 500;
change it to return HTTP 400: in internal/api/plugin.go where errors.As(err,
&httpErr) && httpErr.StatusCode == http.StatusBadRequest is checked, replace
c.JSON(http.StatusInternalServerError, ...) with c.JSON(http.StatusBadRequest,
...) for both the case that parses errResp.Message and the fallback that returns
"invalid request", so the function returns a 400 response using
NewErrorResponseWithMessage containing the plugin-provided message or the
fallback text.

In `@plugin/errors.go`:
- Around line 11-21: The ValidationError.Error method (and Unwrap) can panic if
the receiver or its Err is nil; update Error() to defensively handle a nil
receiver and nil e.Err by returning a safe string (e.g., empty string or the
Message) when nil, and update Unwrap() to return nil if receiver is nil; ensure
NewValidationError remains unchanged but mention ValidationError, Error(), and
Unwrap() so you modify those methods to perform nil checks before dereferencing
e.Err.