[feature](fe) Add current_version and version_long functions

[tonywasson]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Add SQL-visible Doris product version functions while keeping version() pinned to the MySQL compatibility string, and align @@version_comment with the richer Doris build identity.

Release note

Expose current_version() and version_long() in FE SQL metadata while preserving version() compatibility.

Check List (For Author)

• Test: Checkstyle and targeted FE test updates
  • Manual test / No need to test (with reason)
• Behavior changed: Yes (adds new SQL functions and updates @@version_comment output)
• Does this need documentation: Yes

[hello-stephen]

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?

[924060929]

/review

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[924060929]

/review

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[zclllyybb]

/review

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