infoschema: fix inserting into a temporary table panics after the database is dropped

[djshow832]

What problem does this PR solve?

Issue Number: close #29255

Problem Summary:
After the database is dropped, the DBInfo cannot be fetched by TemporaryTableAttachedInfoSchema, so ast.TableName.DBInfo is set to nil. However, many places rely on DBInfo, especially DML. This will lead to panic.

What is changed and how it works?

Retain DBInfo in TemporaryTableAttachedInfoSchema. TemporaryTableAttachedInfoSchema.SchemaByTable returns the DBInfo even when the database is dropped, as long as the temporary table exists.

SchemaByName still returns nil when the database is dropped. I've checked the usage of SchemaByName and found it's safe so far, but there's a potential risk that future usages call SchemaByName instead of SchemaByTable to fetch the DBInfo when checking the existence of a temporary table.

This PR does not involve any release notes because it's a new feature, which is not documented yet.

Check List

Tests

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

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• lcwangchao
• tiancaiamao

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[djshow832]

I encountered an unstable test #29264 and I'm trying to run it again.

[djshow832]

/run-check_dev_2

[tiancaiamao]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b68aee5

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