Warn users about lone dolt_checkout() in dolt sql -q

[kevglynn]

Summary

• When dolt sql -q "call dolt_checkout('branch')" is run as a single statement, print a yellow warning to stderr explaining that the branch change only persists for the current session
• Multi-statement queries (e.g. call dolt_checkout('branch'); select * from t) do NOT trigger the warning, since combining dolt_checkout() with other statements in a single invocation is valid usage
• Suggests using dolt checkout <branch> instead for persistent branch changes

Changes

• go/cmd/dolt/commands/sql.go: Added warnIfLoneDoltCheckout() and isLoneDoltCheckoutCall() — called from queryMode before executing the query
• go/cmd/dolt/commands/sql_test.go: Added TestIsLoneDoltCheckoutCall with 11 test cases (single call, uppercase, mixed case, -b flag, different procedures, multi-statement, empty, bad syntax)
• integration-tests/bats/sql-checkout.bats: Added 2 BATS integration tests — one confirming the warning appears for a lone checkout, one confirming it does NOT appear with multiple statements

Test plan

• go test -run TestIsLoneDoltCheckoutCall ./cmd/dolt/commands/ — all 11 cases pass
• Manual E2E verification: warning appears for lone dolt_checkout(), does not appear for other procedures or multi-statement input
• CI: BATS sql-checkout suite

Closes #6876

[zachmu]

Hi @kevglynn, thanks for the contribution. Sorry for the delay in response!

I like this idea, but the implementation requires parsing the query string an additional time. We already parse all queries in given to sql -q, so this warning logic should take advantage of that fact.

Please make that change and mention me when you have, and I'll take another look for merging.

[kevglynn]

@zachmu Thanks for the review! Good catch — I've refactored the implementation to eliminate the redundant parse.

What changed:

• Removed isLoneDoltCheckoutCall() and warnIfLoneDoltCheckout() which called sqlparser.Parse() separately
• Added a batchExecInfo struct that execBatchMode populates as it processes each already-parsed statement
• execBatchMode now accepts an optional *batchExecInfo parameter — when non-nil, it tracks statement count and whether any statement is a CALL dolt_checkout(...) using isDoltCheckoutCall() on the sqlparser.Statement that ParseWithOptions already produced
• After the loop, if stmtCount != 1, isLoneDoltCheckout is cleared — so multi-statement input never triggers the warning
• queryMode (the -q entry point) passes a &batchExecInfo and checks the result after execution
• Other callers (Exec batch path, debug.go) pass nil — zero behavior change for them

No extra parse. Same observable behavior. Unit tests and BATS tests still pass.

[kevglynn]

@zachmu Rebased onto latest main — the refactored implementation (using batchExecInfo to piggyback on the existing parse in execBatchMode, no redundant parse) is ready for another look whenever you have a chance. Thanks!