fix(swagger): Add dedicated endpoint for contents-ext root directory access

[rickyma]

The contents-ext API has two routes - one for root directory and one for
specific paths, but only the path-specific endpoint was documented in Swagger.
This caused validation errors in auto-generated clients and Swagger UI when
trying to access the root directory.

Changes:

• Add /repos/{owner}/{repo}/contents-ext endpoint for root directory access
• Keep existing /repos/{owner}/{repo}/contents-ext/{filepath} for path access
• Both endpoints share the same implementation but have different Swagger docs

Fixes validation errors when using auto-generated API clients and Swagger UI
to access repository root contents via the contents-ext endpoint.

[rickyma]

@wxiaoguang
I found out the swagger json is auto-generated. We have to add a new endpoint in the backend.
Or maybe we support passing / through filepath?

[wxiaoguang]

Or maybe we support passing / through filepath?

No because there is no / in git path.

[wxiaoguang]

merged into Support getting lastCommitMessage using contents-ext API #34904

[rickyma]

Thanks. Can't wait to apply the PR #34904 :)