[fix](Job)Fix NPE in InsertTask when task is canceled

[CalvinKirs]

When an insert task is canceled during execution, the shared ctx object may be set to null. Currently, after command.runWithUpdateInfo(...), the code directly accesses ctx.getState(). If ctx is null or its state is null, this leads to a NullPointerException (NPE).

Solution

• Cache the ctx reference to a local variable to avoid race conditions.
• Check both ctx and ctx.getState() for null before accessing.
• If the task was canceled or the state is null, safely return without throwing an exception.
• Maintain existing behavior: if the task completes with a non-OK state, still throw JobException.

Impact

• Prevents NPE when a task is canceled during execution.
• Makes the insert task more robust in concurrent cancel scenarios.

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[Thearas]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[morrySnow]

add test case