Harden Echo AI provider request and error handling

[aapomm]

Summary

Improves how the Dradis Echo AI provider integration handles requests and errors (relates to #1641).

• Generic error messaging. InteractionJob previously broadcast the upstream provider's raw response/error to the browser. It now logs the detail server-side and shows the user a generic "Something went wrong." message.
• Consistent Anthropic endpoint. Provider::Anthropic now treats address as a base URL and appends /messages, matching how the OpenAI and Gemini providers build their request URLs. delete_suffix('/messages') keeps existing providers (and full-URL entries) working.

Testing steps

1. Configure a provider whose address points at an endpoint that returns an error (any non-2xx).
2. Run a Dradis Echo interaction on a project.
3. Expected: the browser shows a generic "Something went wrong." message; the full error appears in the server log.
4. Confirm Anthropic requests resolve to .../v1/messages whether the address is set to the base URL or the full endpoint.

Other Information

Full dradis-echo suite passes (101 examples), rubocop clean.

I assign all rights, including copyright, to any future Dradis work by myself to Security Roots.

Check List

• Added a CHANGELOG entry
• Commit message has a detailed description of what changed and why.

[aapomm]

Closing in favor of #1648