fix: preserve root task input across clarification turns

[vinitsonawane45]

Summary

Fixes #7384: Preserve the original task input when a root LlmAgent(mode="task") asks a clarification question and then resumes.

Root cause

Root task agents used a fixed node-path-based isolation scope, allowing separate root task invocations in the same session to share a scope.

When a root task resumed after clarification, the original user message could not be recovered through the existing function-call-based reconstruction because root tasks do not have a delegating function call.

Additionally, nested workflow scopes contain @ characters, so checking only for @ could incorrectly treat a nested scope as a root task scope.

Fix

• Give each root task invocation its own isolation scope using target_node_name@ctx.invocation_id.
• Recover the original root user message using the invocation ID encoded in the root scope.
• Restrict root-scope reconstruction to scopes without /, avoiding incorrect matching of nested workflow-node paths.
• Preserve existing isolation behavior for delegated task nodes.

Tests

• Added regression coverage for root-task clarification and resumption.
• Added a four-turn test covering two sequential root tasks asking separate clarification questions in the same session, ensuring their inputs do not leak across tasks.
• Added assert len(parts) == 2 to catch duplicated clarification answers.
• Verified that the original task input and clarification answer are preserved when the task resumes.

Validation

• Focused workflow tests: 3 passed.
• Workflow task API test file: 15 passed, 1 xfailed.
• A2A v1 clarification/resumption integration test: 1 passed.
• Targeted content tests: 2 passed.
• Ruff, Pyink, and git diff --check: passed.

The broader unit suite also encountered Windows-specific failures in unrelated Cloud Run and Bash tests; the entire suite should not be described as passing.

Isolation behavior

Root task invocations use per-invocation scopes. Clarification and resumption within the same task are preserved, while separate root task invocations do not share intermediate task execution history.

Fixes #7384