fix(extensions): support optional source on extension instances and add ensureInstanceSpec (#11011)

* fix(extensions): support optional source on extension instances and add ensureInstanceSpec

### Description
- Updates `ExtensionConfig` interface to make `source?: ExtensionSource` optional, reflecting runtime API behavior for published extension instances.
- Adds `ensureInstanceSpec` in `extensionsHelper.ts` to fetch an extension version's specification on demand when `instance.config.source?.spec` is absent.
- Adds defensive guards when accessing `instance.config.source?.spec` in `secretsUtils.ts` and `export.ts`.

### Scenarios Tested
- Added unit tests in `src/extensions/export.spec.ts` for `ensureInstanceSpec`:
  - Returns instance unchanged when spec is already present.
  - Fetches spec on demand from the Extension publisher API when missing.
- Verified TypeScript compilation and ESLint checks.

### Sample Commands
- `npm test`

* fix(extensions): address review feedback on optional chaining and fallback extensionRef

- Use optional chaining for instance.config?.source?.spec in getManagedSecrets.
- Fall back to top-level instance.extensionRef and instance.extensionVersion in ensureInstanceSpec.
- Add unit test verifying top-level instance.extensionRef spec fetch.

* fix(extensions): refactor ensureInstanceSpec with early returns and error bubbling

- Remove silent error suppression; allow errors from getExtensionVersion to bubble up.
- Flatten control flow with early returns.
- Populate real source metadata from ExtensionVersion instead of placeholder fields.
- Update unit tests.

* fix(ext): address PR review comments

- Rename DEFAULT_FUNCTION_REGION to FUNCTION_DEFAULT_REGION in functionsEnvFromInstance.
- Use ref.version from parsed extensionRef if top-level extensionVersion is unset in ensureInstanceSpec.
- Co-locate ensureInstanceSpec unit tests in extensionsHelper.spec.ts.