Refactor rebuild test harnesses into a shared core with generic and Deep Agents Code profiles #8359

Objective

Replace two overlapping rebuild harness implementations with one shared core and two named profiles.

Evidence

• test/helpers/rebuild-flow-harness.ts is 872 lines.
• test/helpers/rebuild-flow-test-harness.ts is 695 lines.
• Both load the rebuild graph, create sessions, install overlapping spies, emulate sandbox state, and restore mocks.
• The older harness owns DCode route and image behavior.
• The newer harness owns broader generic recovery, credential, messaging, and prepared-image behavior.
• test/helpers/rebuild-flow-test-support.ts already owns part of the reusable type and session layer.

Scope

• Extract one core for shared module loading, session creation, common spies, sandbox state, and cleanup.
• Add only two profiles: generic and Deep Agents Code.
• Migrate all current harness consumers.
• Remove the superseded duplicate implementation.
• Keep DCode-only image and route mocks out of the generic profile.
• Keep generic credential, messaging, and prepared-image mocks from becoming mandatory DCode setup.
• Do not change production source, supported behavior, test titles, or assertions.

Relationships

• Parent Epic: Consolidate test support and enforce test-suite boundaries #8345
• Blocked by:
  • Consolidate the duplicate DCode test helper #8284 — establishes one DCode rebuild-helper and probe-fixture owner.
  • Reuse canonical rebuild recovery and environment test fixtures #8353 — removes duplicate recovery and environment fixtures.
  • Make six rebuild-flow case modules ordinary Vitest suites #8354 — makes all six rebuild case modules ordinary suites before harness migration.
• Blocks: None.
• Start condition: Do not start until GitHub shows zero active blockers.

This is not a good first issue: it changes mock lifecycle across the rebuild suite.

Acceptance criteria

• Common session, environment, module-loading, and mock lifecycle logic has one implementation.
• Every rebuild suite selects the generic or DCode profile explicitly.
• Profile-specific mocks remain isolated.
• Existing test titles and collected-test counts remain unchanged.
• Repository size, complexity, and project checks pass.
• The change has a negative net LOC delta.

Validation

npx vitest run --project cli src/lib/actions/sandbox/rebuild*.test.ts
npm run typecheck:cli
npm run checks:repository