Fix truncated Grape routes in Cells routing snapshot

What does this MR do and why?

API::API.routes memoizes its result, and grape-swagger reads it from add_open_api_documentation! (lib/api/api.rb:426) while only 185 of the eventual 358 endpoints are mounted. gitlab:cells:routes:generate inherited that partial memo, so it collected 1228 of 2142 Grape routes.

As a result config/routing/gitlab_routes.json was missing 598 API path templates - every route mounted after the OpenAPI block, for example:

/api/:version/projects/:id/boards
/api/:version/admin/sidekiq/queues/:queue_name
/api/:version/projects/:id/issues/:noteable_id/discussions

Paths absent from the snapshot are paths we could potentially misroute.

This MR calls API::API.compile! and API::API.reset_routes! before reading the route table, then regenerates the snapshot. reset_routes! is the line that fixes it (the endpoint tree already holds 2142 leaves before compiling); compile! guarantees every mount is materialised and mirrors what Gitlab::RequestEndpoints.all_api_endpoints already does.

Snapshot size goes from 4445 to 5043 entries. The change is purely additive:

added templates          598
removed templates          0
changed existing entries   0

Checklist notes

• Changelog: omitted. This is development tooling; no user-facing behaviour changes.
• Tests: spec/tasks/gitlab/cells/routes_rake_spec.rb stubs RoutesSnapshot.new and only asserts that both route tables reach the constructor, so it neither caught nor needs to catch this. A test that would catch it has to boot the full route table, which the gitlab:cells:routes:updated_check CI job already does.
• Documentation: no behaviour, API surface, configuration or UI change.
• Screenshots: not applicable, no UI change.
• Database / Application Security review: not applicable.
• Translation: no user-facing strings.
• Feature flag: not applicable, the task is run on demand and in CI.

References

This also tackles the missing internal APIs in the snapshot gitlab-com/gl-infra/tenant-scale/cells-infrastructure/team#786 (closed)

Screenshots or screen recordings

Not applicable, no UI change.

Before	After
4445 snapshot entries, 956 unique Grape paths	5043 snapshot entries, 1554 unique Grape paths

How to set up and validate locally

1. Check out master and confirm the truncation:

   RAILS_ENV=test CI=true GITLAB_CONFIG=$(pwd)/config/gitlab.yml.example \
     bundle exec rails runner 'puts API::API.routes.count'
   # => 1228

2. Confirm the full table is larger:

   RAILS_ENV=test CI=true GITLAB_CONFIG=$(pwd)/config/gitlab.yml.example \
     bundle exec rails runner 'API::API.compile!; API::API.reset_routes!; puts API::API.routes.count'
   # => 2142

3. Check out this branch and regenerate; the file must not change:

   bundle exec rake gitlab:cells:routes:generate
   # => Generated 5043 routes at config/routing/gitlab_routes.json
   git diff --stat -- config/routing/gitlab_routes.json

4. Confirm the sync check passes:

   bundle exec rake gitlab:cells:routes:updated_check

MR acceptance checklist

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