[STG-1802] fix: abort immediately for already-aborted agent signals

[shrey150]

Summary

• throw AgentAbortError before agent setup when the supplied signal is already aborted
• deflake the abort-signal integration test by removing Browserbase init from the immediate-abort case
• add a changeset and unit coverage for the fast path

Verification

• pnpm exec prettier --check .changeset/quiet-cameras-knock.md packages/core/lib/v3/v3.ts packages/core/tests/integration/agent-abort-signal.spec.ts packages/core/tests/unit/browserbase-session-accessors.test.ts
• pnpm --filter @browserbasehq/stagehand typecheck
• pnpm --filter @browserbasehq/stagehand build:esm
• pnpm --filter @browserbasehq/stagehand test:core -- packages/core/dist/esm/tests/unit/browserbase-session-accessors.test.js
• node --import tsx -e "already-aborted signal smoke test"

Linear: https://linear.app/browserbase/issue/STG-1802/abort-immediately-for-already-aborted-agent-signals

---

Summary by cubic

Abort agents immediately when given an already-aborted signal, throwing AgentAbortError before any setup. Aligns with Linear STG-1802 and removes flakiness in the immediate-abort path for @browserbasehq/stagehand.

• Bug Fixes
  • Added early abort check in V3 execute path (throwAgentAbortIfAborted), using signal.reason when present.
  • Deflaked tests by skipping Browserbase init for the immediate-abort case and added unit coverage for the fast path.

^{Written for commit b84f012. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant App as Client Application
    participant V3 as V3 Core (Stagehand)
    participant BB as Browserbase / Session
    participant LLM as LLM Client

    Note over App, LLM: Agent Execution Flow (with AbortSignal)

    App->>V3: agent.execute({ instruction, signal })
    
    V3->>V3: NEW: throwAgentAbortIfAborted(signal)
    
    alt NEW: Signal is already aborted (Fast Path)
        V3-->>App: throw AgentAbortError (reason)
        Note right of V3: Execution stops before initializing resources
    else Signal is active
        V3->>V3: validateExperimentalFeatures()
        
        opt CHANGED: If not initialized (Integration Tests)
            V3->>BB: init() / Session Setup
            BB-->>V3: session ready
        end
        
        V3->>LLM: setup & query handler
        
        loop Agent Loop
            V3->>LLM: Generate next action
            LLM-->>V3: action
            V3->>BB: Perform action on Page
            
            opt Signal aborted during execution
                V3-->>App: throw AgentAbortError
            end
        end
        
        V3-->>App: AgentResult / Stream
    end

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