[refine](function) split monolithic function_string.h into domain-specific files

[Mryange]

What problem does this PR solve?

function_string.h was a 5393-line monolithic header containing ~40 function class implementations with heavy dependencies (pugixml, ICU, Boost.Locale, digest libs, etc.). Every translation unit including it pulled in all dependencies, causing slow compilation and unnecessary rebuilds.

This PR splits it into 9 domain-specific files. 6 files with no external consumers are converted to standalone .cpp with their own registration functions. 3 files with external consumers remain as .h.

Converted to .cpp (6 files):

• function_string_basic.cpp — Strcmp, Substring, Left, Right, NullOrEmpty
• function_string_mask.cpp — Mask, MaskPartial
• function_string_search.cpp — LocatePos, SplitPart, SubstringIndex, SplitByString, CountSubString
• function_string_digest.cpp — SM3, MD5, SHA1, SHA2
• function_string_url.cpp — ExtractURLParameter, ParseUrl, UrlDecode, UrlEncode
• function_string_misc.cpp — AutoPartitionName, RandomBytes, ConvertTo, IntToChar, NgramSearch, Translate, XPathString, MakeSet, ExportSet, Crc32, UnicodeNormalize

Kept as .h (3 files):

• function_string_concat.h — used by column_string_test.cpp
• function_string_format.h — used by function_money_format_test.cpp
• function_string_replace.h — used by function_reverse.h, function_sub_replace_test.cpp

Other changes:

• function_string.cpp — removed original header include, calls 6 sub-registration functions via extern declarations
• Removed unnecessary function_string.h includes from partition_transformers.h and function_split_by_regexp.cpp
• Updated test files to include specific sub-headers
• Fixed missing block.h include in function_array_reverse.h
• Fixed pre-existing missing vexpr_context.h include in viceberg_merge_sink.cpp

The original function_string.h is deleted.

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

[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?

[Mryange]

run buildall

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 92.84% (3617/3896) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.94% (26756/37192)
Line Coverage	54.91% (283373/516096)
Region Coverage	51.78% (233966/451813)
Branch Coverage	53.31% (101326/190057)

[Mryange]

/review

[github-actions]

I found 1 issue that should be addressed before merge.

1. Goal of the task: splitting function_string.h into smaller translation units does reduce include fanout, but the refactor is not fully self-contained yet because one of the new files still relies on an implicit RE2 declaration from the global PCH.
2. Modification size/focus: the change is otherwise focused on the string-function split and related include cleanup.
3. Concurrency: not applicable; I did not find new threading or locking changes in this PR.
4. Special lifecycle/static initialization: no new lifecycle or static-init issues identified in the modified paths.
5. Configuration: no config changes.
6. Compatibility/incompatible changes: no FE/BE protocol or storage-format compatibility issue identified.
7. Parallel code paths: registration/alias coverage for the split string functions looks consistent overall; I did not find a missing runtime registration in the reviewed paths.
8. Special conditional checks: no new problematic conditional logic identified beyond the issue below.
9. Test coverage: there are no new tests, and for a refactor like this the build itself is the key coverage. The missing direct include means the split file is not robust outside the current PCH build setup.
10. Observability: not applicable.
11. Transaction/persistence: not applicable.
12. Data writes/modifications: not applicable.
13. FE-BE variable passing: not applicable.
14. Performance: the intended compile-time improvement makes sense; I did not identify a runtime-performance regression.
15. Other issues: see the inline comment.

Summary opinion: the refactor is directionally good, but the new function_string_misc.cpp translation unit should add its direct RE2 dependency before merging so the split remains self-contained and build-robust.

[github-actions]

FunctionAutoPartitionName::_auto_partition_type_of_range() still instantiates RE2 later in this file, but this new TU no longer includes re2/re2.h directly. After the split, it only builds because the BE target's global PCH happens to pull RE2 in transitively. That makes the refactor brittle: building this source without the PCH path will fail with an unknown RE2 type. Please add a direct #include <re2/re2.h> here so the new file is self-contained.

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.