Declare path parameters in API files (Batch 1)

What does this MR do and why?

Adds requires / optional declarations for path placeholders in 32 API files. Without these declarations, the generated OpenAPI v3 spec emits schema: {} (no type information) for each path placeholder — valid OpenAPI 3.0 but missing the type metadata the published API reference depends on.

Each declaration uses the type appropriate for that file's resource:

• [String, Integer] for IDs that accept URL-encoded paths (projects/groups/namespaces)
• Integer for numeric DB IDs
• String for slug-style identifiers

Each fixed file is also removed from .rubocop_todo/api/path_parameter_declaration.yml, so the API/PathParameterDeclaration cop becomes active on those files. The companion batch 2 is stacked on this MR and removes the remaining 32 entries (and the now-empty todo file).

The diff to doc/api/openapi/openapi_v3.yaml (and the companion v2 spec) is the regenerated output of bin/rake gitlab:openapi:v3:generate / :v2:generate; no manual edits.

YAML reduction: 474 → 421 synthesized path parameters in this MR (-53).

How to set up and validate locally

grep -c "schema: {}" doc/api/openapi/openapi_v3.yaml
# => 421 after this MR (was 474 on master)

bundle exec rubocop --only API/PathParameterDeclaration lib/api ee/lib/api
# => 0 offenses

bin/rake gitlab:openapi:v3:generate
bin/rake gitlab:openapi:v2:generate
# Both should report "up to date" after running.

References

Related to: #593537 (closed)