[improvement](fe) Add --fast-compile option to run-fe-ut.sh

[englefly]

When --fast-compile is specified, use tools/fast-compile-fe.sh for incremental compilation instead of a full mvn build. This avoids recompiling unchanged FE code and speeds up the edit-test cycle.

• Checks target/classes completeness and doris-fe.jar existence (references fast-compile-fe.sh check_env)
• Counts changed files; falls back to full build if > 1000
• Falls back to full build on any fast-compile failure
• Limits surefire:test scope to fe-core via -pl to avoid unnecessary reactor processing of other modules
• Without --fast-compile, behaviour is unchanged for QA compatibility

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

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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

[hello-stephen]

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