Support staging new tables with dolt add -p

[nullun]

When using 'dolt add -p' to stage rows from a new table (one that exists in working but not in staging), the workspace table UPDATE mechanism previously failed with 'table not found' because GetTableWriter looked for the table in the staging root where it didn't exist yet.

This change adds ensureTableExistsInStaging() which:

1. Checks if the table exists in staging (fast path - no-op)
2. If not, checks if it exists in working
3. If yes, creates an empty table in staging with the same schema
4. Updates the session state to reflect the new staging root

The table is created empty (not copied with data) because 'dolt add -p' allows partial staging - each row selected by the user will be inserted individually into the staging table via the workspace table UPDATE.

Also adds unit tests and BATS integration tests for the new functionality.

Example usage: https://gist.github.com/nullun/e88cb2dab9568c6612c98d415a4a2efd

[jycor]

LGTM! Thanks for the contribution

[jycor]

Looks like there are some failing tests.

[nullun]

Hmmm, just got the email about the test failing again.

2026-03-02T18:50:45.2740284Z # `[[ "$output" =~ "| true | 1 | alice |" ]] || false' failed

Not sure I understand why off the top of my head, unless the formatting is different somehow? I'll take a look in the morning if that's okay.

[jycor]

Hmm... that should be right. It's possible it's something silly like a missing/extra space.
Tomorrow morning is totally fine, take your time.

The individual BATS statements can just be run the in the shell if that helps.

[nullun]

Should have fixed it. I didn't realise there was a different when using remote-engine, which were what was failing. So my previous checks for "true" and "false" were actually "1" and "0". I thought simply using -r csv would make the output identical on both, but realised that wasn't fixing the actual problem, so now the SQL query itself will convert 1/0 to true/false.

The previously failing tests are now working, but let me know if you want me to tidy anything up or change it to be another way.
Bats tests (ubuntu-22.04)

ok 61 add-patch: new table partial staging

Bats tests

Using remote engine for tests
ok 61 add-patch: new table partial staging

[jycor]

Everything looks good. Thanks for the contribution!