…#2370)

### What and why?

Re-running `instrument` on a serverless-init sidecar target (Azure Container App) must be a true no-op. Previously a re-instrument could drop provider-populated sidecar fields (e.g. `resources`) and re-append the shared volume mount, producing config drift. This makes the sidecar merge idempotent and enables APM on the Container App sidecar.

Split out from the Container App e2e PR (#2364) so the production behavior change is reviewed on its own. Stacked on #2333.

### How?

- `packages/base/src/helpers/serverless/common.ts`: when a sidecar container already exists, merge the new config **onto** it -- preserving existing `resources` and env vars, and deduping the shared volume mount via a new `ensureSharedVolumeMount` helper -- instead of replacing it wholesale.
- `packages/plugin-container-app/src/common.ts`: set `DD_APM_ENABLED=true` so the sidecar enables APM.
- Updates the affected unit tests and the instrument snapshot.

### Test plan

- `yarn test` for `packages/base` serverless helpers and `packages/plugin-container-app` (60 tests pass).
- End-to-end idempotency is asserted by the Container App e2e suite in #2364.

### Review checklist

- [x] Feature or bugfix MUST have appropriate tests (unit, integration)