fix(gh-copilot): trim enterprise/organization slugs to fix connection validation failure

[bujjibabukatta]

Summary

Fixes #8991 — GitHub Copilot connector's "Test Connection" fails with
either enterprise or organization is required even when a valid Enterprise
Slug is filled in and Organization is left blank.

Root Cause

validateConnection() checked Organization for emptiness without trimming
it, while HasEnterprise() already trimmed Enterprise. This mismatch meant
a slug with incidental whitespace (e.g. copy/pasted from GitHub) could be
treated as "present" in one check and "empty" in another. A second, separate
copy of this same check in connection_test_helper.go also had a different
error message, showing the two had drifted out of sync.
The frontend's IConnectionAPI/IConnection types were also missing the
enterprise field, and the test/testOld API payload types didn't
include it either — a type-safety gap worth closing alongside this fix.

Fix

• models/connection.go — trim Organization/Enterprise in Normalize()
• api/connection.go — trim Organization in validateConnection()
• service/connection_test_helper.go — align error message with the above
• api/connection_test.go — added regression tests going through the real
  JSON decode path (hyphenated slug, whitespace-only org)
• config-ui/src/types/connection.ts — added missing enterprise field
• config-ui/src/api/connection/index.ts — added enterprise to payload types