refactor(forge): share one REST client between GitHub, Gitea and GitLab pagination

[BryanFRD]

Closes #1285

gitea.rs was 36.7% duplicated from github.rs (Sonar): pagination, the three release calls and the three comment calls, identical except for headers, the page-size parameter and the error codes. GitLab had a third copy of the pagination loop.

src/forge/rest.rs now holds a RestClient built per forge with its auth header, extra headers and page-size parameter (RestClient::github, ::gitea, ::gitlab). It owns pagination, create_release, find_draft_release, publish_release, find_comment, create_comment and update_comment. Each forge keeps its own context messages and error codes by wrapping the call, so the error chain users see is unchanged.

Two small differences worth knowing:

• Pagination now builds its query with ureq's query, giving the same ?per_page=…&page=… (or ?limit=… on Gitea) URL as before.
• On GitHub, the comment calls now also send X-GitHub-Api-Version, which the release and pagination calls already did. That is the documented header; the inconsistency was accidental.

percent_encode_path moves from config::loader_js to a neutral crate::uri module, since both forges and the JS loader use it (raised in the reviews of #1267 and #1269).

Forge and config tests pass (518), including the fake-server API tests for each forge.

[ferrfleet]

Behaviour matches the old per-forge code: same headers (GitHub comment calls now also send X-GitHub-Api-Version, which is harmless), same page sizes, same error contexts and codes. Switching to .query() for paging is an improvement. Test and Coverage were still running when I reviewed.

Nit: RestClient::gitlab (src/forge/gitlab.rs:54) is only used for pagination. Its repo_url is built and then never read, and the client still exposes create_release, create_comment, update_comment and find_comment. Those methods use GitHub/Gitea paths and payloads (/issues/..., body rather than description), so a later caller would get a client that looks valid and hits the wrong GitLab endpoints. I'd move the pagination loop into a free paginate(request_fn, ...) that all three forges share, and keep RestClient for GitHub and Gitea only. If you'd rather keep the GitLab constructor, at least add a doc comment saying only paginated_json_array applies.