Container Assist: support a single shared manifest across multiple services

[Tatsinnit]

Context

Reviewer feedback on #2145 (thread):

some repos can have a single manifest for all services. are we going to support that?

Current behaviour

promptForManifestSelection (in src/commands/aksContainerAssist/workflowGenerator.ts) asks the user for a manifest path per service. A shared manifest works today but the user has to re-select the same file N times — functional, just clunky.

Proposal

After the first service's manifest is chosen, surface a quick pick:

• Apply this manifest to all remaining services — auto-fills the same path for the rest, with a confirmation summary at the end.
• Choose individually — current behaviour.

Selection should also be revisitable (e.g. an "Edit manifest paths" step before workflow generation).

Alternative UX to consider

• Multi-select up front: present manifests once, let the user assign services to manifests (table-style picker). Higher implementation cost.
• Detect identical manifest across all detected services and pre-select with confirm. Lower friction, no new UI.

Acceptance criteria

• Single-manifest-for-all path requires ≤ 2 prompts regardless of service count.
• Per-service overrides still possible.
• The shared manifest is correctly rendered as a multi-line block scalar in each deploy-<svc> job (re-uses the reindentBlockScalar helper added in Implements Azure/vscode-aks-tools project #994: #2145).
• Add a unit test covering "same manifest applied to N services".

Follow-up to #2145.