[fix](function) prevent count_substrings tail overmatch

[officialasishkumar]

What problem does this PR solve?

Issue Number: close #62768

Related PR: None

Problem Summary:

count_substrings scanned candidate positions even when the remaining suffix was shorter than the search pattern. Because the implementation uses memcmp_small_allow_overflow15, that could count a tail position where the full pattern does not fit, for example count_substrings("ccc", "cc").

This PR limits comparisons to positions where the full pattern fits and keeps the existing not-found distance contract used by the caller.

Release note

Fix count_substrings tail-boundary matching for non-overlapping substring counts.

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. count_substrings no longer counts a match when the full pattern does not fit in the remaining suffix.

• 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

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