planner: remove intest.Assert on missing schema in memtable infoschema extractor

[Copilot]

What problem does this PR solve?

Problem Summary:

findSchemasForTables already handles SchemaByID misses (possible during concurrent DDL), but it still had intest.Assert(ok) immediately before the defensive branch. In intest/enableassert builds this could still panic, defeating the intended graceful handling.

What changed and how does it work?

• Targeted fix
  • Removed the unconditional intest.Assert(ok) in pkg/planner/core/memtable_infoschema_extractor.go.
• Behavior after change
  • When SchemaByID returns !ok, the code now consistently follows the existing warning-and-continue path instead of panicking in assert-enabled test builds.
• Scope
  • Single-line behavioral adjustment for the referenced review comment; no additional logic changes.

dbInfo, ok := is.SchemaByID(tbl.DBID)
if !ok {
    logutil.BgLogger().Warn("schema not found for table info",
        zap.Int64("tableID", tbl.ID), zap.Int64("dbID", tbl.DBID))
    continue
}

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.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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