Refactor handling of OAuth client tokens

[NobodysNightmare]

Previously the fetching and updating of OAuthClientToken was left as an implementation detail of specific integrations, even though the concept of OAuthClient and OAuthClientToken were defined in the core and even their creation is already handled through a core class (the ConnectionManager).

This PR introduces the OAuthClients::TokenFetcher that serves as the new main interface to obtain the access token of an OAuthClientToken. A relevant difference compared to the previous approach is that we refresh access tokens based on their time-to-live (i.e. expires_in), not based on receiving an error response and then retrying.

Ticket

https://community.openproject.org/wp/SI-236

[github-actions]

Warning

Flaky specs

• rspec ./spec/features/workflows/edit_spec.rb[1:7:8]

🤖 Ask Copilot to investigate

Copy the prompt below into a new comment on this PR to delegate the investigation to GitHub Copilot. It will look into the flakiness and open a separate pull request with you as reviewer.

@copilot The following spec(s) are flaky in CI (first seen on PR #24684, linked for reference only):

- `rspec ./spec/features/workflows/edit_spec.rb[1:7:8]`

Treat this as a standalone task, unrelated to PR #24684. Create a new branch from origin/dev and open a new pull request targeting dev — do not stack it on PR #24684 or reuse that branch.

Follow the playbook in docs/development/testing/handling-flaky-tests/README.md to find the root cause and fix the underlying race — do not skip, delete, or weaken the spec to make it pass; disabling is a last resort per the playbook, and only with a bug ticket. Verify the fix by running the spec(s) repeatedly (e.g. `script/bulk_run_rspec --run-count 10`).

If you cannot reproduce the flake or are not confident in a fix after reasonable investigation, do not fabricate a change or skip the spec to force CI green. Instead, leave the pull request in draft and document what you tried, the suspected cause, and any leads in its description, then assign @NobodysNightmare to take over.

Once the fix is verified, title the PR after the spec(s) it fixes, and use the PR description to explain the root cause, how the change resolves it, and the before/after results. Label the PR `flaky-spec`, assign @NobodysNightmare, and request a review from @NobodysNightmare.
On every commit, set @NobodysNightmare as the sole co-author with a `Co-authored-by:` trailer (use their GitHub no-reply email so it links to their account), so it is traceable who dispatched the fix.

[github-actions]

Warning

Flaky specs

• rspec ./modules/backlogs/spec/features/inbox_column_spec.rb[1:7:1]
• rspec ./spec/features/work_packages/details/inplace_editor/version_editor_spec.rb[1:1:2]

🤖 Ask Copilot to investigate

Copy the prompt below into a new comment on this PR to delegate the investigation to GitHub Copilot. It will look into the flakiness and open a separate pull request with you as reviewer.

@copilot The following spec(s) are flaky in CI (first seen on PR #24684, linked for reference only):

- `rspec ./modules/backlogs/spec/features/inbox_column_spec.rb[1:7:1]`
- `rspec ./spec/features/work_packages/details/inplace_editor/version_editor_spec.rb[1:1:2]`

Treat this as a standalone task, unrelated to PR #24684. Create a new branch from origin/dev and open a new pull request targeting dev — do not stack it on PR #24684 or reuse that branch.

Follow the playbook in docs/development/testing/handling-flaky-tests/README.md to find the root cause and fix the underlying race — do not skip, delete, or weaken the spec to make it pass; disabling is a last resort per the playbook, and only with a bug ticket. Verify the fix by running the spec(s) repeatedly (e.g. `script/bulk_run_rspec --run-count 10`).

If you cannot reproduce the flake or are not confident in a fix after reasonable investigation, do not fabricate a change or skip the spec to force CI green. Instead, leave the pull request in draft and document what you tried, the suspected cause, and any leads in its description, then assign @NobodysNightmare to take over.

Once the fix is verified, title the PR after the spec(s) it fixes, and use the PR description to explain the root cause, how the change resolves it, and the before/after results. Label the PR `flaky-spec`, assign @NobodysNightmare, and request a review from @NobodysNightmare.
On every commit, set @NobodysNightmare as the sole co-author with a `Co-authored-by:` trailer (use their GitHub no-reply email so it links to their account), so it is traceable who dispatched the fix.

[NobodysNightmare]

Follow me to #24726