planner: remove intest.Assert(ok) that could panic during concurrent DDL

[Copilot]

What problem does this PR solve?

Issue Number: ref #69676

intest.Assert(ok) in memtable_infoschema_extractor.go could panic in intest/enableassert builds when SchemaByID returns !ok. This code path is legitimately reachable during concurrent DDL (schema version change between table list and lookup), so the assertion is incorrect and undermines the defensive !ok handling added immediately after it.

What changed and how does it work?

• pkg/planner/core/memtable_infoschema_extractor.go: Removed intest.Assert(ok) — the !ok branch already handles this case gracefully (logs a warning and continues). Also removed the now-unused intest import.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.
    The change only removes an assertion that was incorrect for a known-valid code path. The defensive !ok handling it guarded is already covered by existing logic.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Details

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[ti-chi-bot]

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Notice: To remove the do-not-merge/needs-linked-issue label, please provide the linked issue number on one line in the PR body, for example: Issue Number: close #123 or Issue Number: ref #456.

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