Handle GitLab ref lookup rejections immediately

[kylos101]

Summary

• await the GitLab repository and ref lookups together so both promises receive rejection handlers immediately
• preserve the existing missing-ref error semantics
• add a regression test using a percent-encoded zero-width character in the missing ref while the repository lookup remains pending

Root cause

The ref lookup started before the repository lookup completed, but its promise was not awaited until afterward. When ref resolution failed first, Node reported an unhandled rejection before the request eventually observed and propagated the same error.

Validation

• yarn workspace @gitpod/server build
• yarn workspace @gitpod/server test:unit -- --grep 'GitlabContextParserUnitSpec'
• pre-commit run --files components/server/src/gitlab/gitlab-context-parser.ts components/server/src/gitlab/gitlab-context-parser.unit.spec.ts
• regression check: the focused test fails with the previous sequential await ordering because it observes the unhandled rejection, and passes with Promise.all()