feat(core): support scoped AI contexts and stateless Test Runner workflows

[xiong35]

Summary

• Add default, per-API, and per-call AI context configuration.
• Resolve contexts using strict override precedence:
  options.context > aiContexts[apiName] > aiContexts.default.
• Add setAIContext() for updating the default or an API-specific context.
• Preserve the deprecated aiActContext compatibility APIs.
• Remove cross-step Test Runner history so workflow steps no longer implicitly receive previous step results.
• Update unit tests, E2E tests, and bilingual documentation.

Context resolution

Only the highest-priority defined context is used:

1. Per-call options.context
2. API-specific aiContexts[apiName]
3. Shared aiContexts.default

Contexts are not merged automatically.

• '' explicitly clears lower-priority context.
• undefined falls back to the next configured level.
• Users can concatenate context strings explicitly when composition is needed.

The selected context remains raw in task and report data and is wrapped in a generic <CONTEXT> block only when building the LLM input.

Backward compatibility

• aiActContext and aiActionContext remain available as deprecated aliases for aiContexts.aiAct.
• setAIActContext() and setAIActionContext() remain available as deprecated aliases for setAIContext('aiAct', context).
• When both a legacy option and aiContexts.aiAct are provided, aiContexts.aiAct takes precedence.

Test Runner behavior

• Remove ctx.history, NodeHistoryEntry, workflow history accumulation, and automatic history injection.
• Workflow steps are now independent and do not implicitly inherit previous step outputs.
• Shared knowledge can still be provided explicitly through Project context or Agent AI contexts.
• The internal planning conversation within a single aiAct call remains unchanged.

Validation

• pnpm run lint
• pnpm run type-check:tests
• pnpm exec nx run-many -t build -p @midscene/core @midscene/test --parallel=2
• pnpm exec nx run-many -t test -p @midscene/core @midscene/shared @midscene/test @midscene/android --parallel=3
• pnpm --filter @midscene/test exec vitest --run --config vitest.e2e.config.ts

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