Scope native artifact uploads to child tasks

[warp-agent-staging]

Description

Fix native upload_artifact calls that resolved a local child association but omitted its ambient task identity from artifact GraphQL requests.

The artifact uploader now requires the resolved task ID for both upload-target creation and confirmation, and sends it through request-local GraphQL header policy. This removes the CLI's shared ServerApi task-ID mutation, so concurrent local children cannot race by overwriting ambient task context. Server authorization behavior is unchanged.

Linked Issue

No linked issue.

Testing

• cargo test -p warp --lib ai::agent_sdk::artifact_upload::tests (16 passed)
• cargo test -p warp_server_client authenticated_graphql_options_can_override_task_id_for_one_request
• cargo test -p warp_server_client base_client::tests::ambient_policy_supports_inherit_override_and_omit
• ./script/format --check
• ./script/check_no_inline_test_modules
• cargo clippy --workspace --exclude warp_completer --all-targets --tests -- -D warnings
• cargo clippy -p warp --all-targets --tests -- -D warnings
• I have manually tested my changes locally with ./script/run — not applicable to this non-UI request-header change.

Cross-platform cloud verification was skipped because the final diff is platform-agnostic Rust/GraphQL header plumbing with no OS- or architecture-specific code, path behavior, native dependencies, ABI, or packaging changes.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Attach native artifact uploads to the correct local child task.

Co-Authored-By: Warp Agent agent@warp.dev

Conversation: https://staging.warp.dev/conversation/bc181c55-ba82-40ae-99f4-76e7b8de461f
Run: https://oz.staging.warp.dev/runs/019ff287-bb8d-7d93-b349-a39904e21727

This PR was generated with Oz.