[r266-tech]

Description

Connection failures now include reqwest's underlying cause in JSON and ordinary table output, so TLS certificate and malformed-HTTP errors can be diagnosed directly.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Fixes #6288.

Type of Change

• Bug fix

Changes Made

Append the source chain in the existing network-error branch and display that message in the connection error card. Add a real CLI regression for both output formats. Rustfmt also reflows surrounding error match arms.

Testing

• New regression fails on the base and passes with the fix.
• 511 CLI tests passed on macOS: 506 unit tests and 5 integration tests.
• Local self-signed TLS fixture: both formats expose the certificate error after the fix; the base hides it.
• Real timeout, invalid-gzip decoding, and unsupported-scheme controls retain their error codes and card titles in both formats.
• Rustfmt and git diff --check passed.

CLI tests used Rust 1.95, RUST_MIN_STACK=16777216, and a temporary CLI-only workspace to avoid unrelated RAGFS dependency resolution; every selected dependency retained its original lockfile version. The complete workspace manifest/lock are restored and unchanged. With the default test stack, an existing help-parser test overflowed before the suite completed. No live service or model was called.

Checklist

• I have performed a self-review of my code.
• I have tested my changes locally.
• The change stays within the reported connection-diagnostics scope.

Hosted CI

All five CLI platform builds passed. The API/CLI job fails during the API phase: test_content_write_empty_content and test_content_write_creates_new_file return 500 after embedding HTTP 401 (52 passed, 1 skipped in that batch). Its setup selected the minimal/no-secrets configuration. The Ubuntu release-base job at e855796c passed; this is not a blanket main-branch failure. The fork's credential-dependent API fixture failure remains unresolved.

[r266-tech]

Closing in favor of #6312, which was opened first and covers the same #6288 connection-diagnostics fix. The CLI integration regression on this branch remains available if useful.