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 Add Container App e2e telemetry contract #2364.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)