Fix insert_all/upsert_all for single character, temporary and three part table names

[Alt70155]

Fixes #1402.

I used AI assistance to narrow down the cause and to check the patch for
regressions, but everything here was verified by running the reproduction script
and the adapter's test suite against a local SQL Server 2019.

upsert_all (and insert_all when it takes the MERGE path) raises
NoMethodError when the MERGE table name pattern cannot match the target, and
returns a wrong name when the target carries an alias. See #1402 for the full
analysis of why the two identifier slots behave that way.

INSERT, UPDATE and FROM already handle three part names; only MERGE was
limited to two.

Fix

Replace the pattern with an explicit identifier that matches either a bracket
quoted or a bare name, joined by up to two dots:

MERGE_TARGET_IDENTIFIER = /(?:\[[^\]]+\]|[a-z0-9_-]+)/i # :nodoc:
MERGE_TARGET_TABLE_NAME = /\A\s*MERGE\s+INTO\s+(#{MERGE_TARGET_IDENTIFIER}(?:\s*\.#{MERGE_TARGET_IDENTIFIER}){0,2})\s+(?:AS|WITH|USING)/i # :nodoc:

Notes on the choices:

• A bracket quoted segment is \[[^\]]+\], so spaces inside brackets are kept
  ([WITH - SPACES]) while a space is no longer allowed in a bare name. That is
  what stops an aliased target being captured as part of the table name.
• The dot separator is \s*\., not \., so MERGE INTO dbo .products keeps
  working. It parses today and I did not want to change that.
• {0,2} allows one, two and three part names, matching the other branches.

Tests

• test/cases/schema_test_sqlserver.rb — 8 cases added to the existing
  MERGE statements block (5 -> 13): bracket quoted and bare names, a single
  character name, a name with spaces inside brackets, two and three part names,
  with and without a WITH (UPDLOCK, HOLDLOCK) hint, and the space before the
  dot case.
• test/cases/adapter_test_sqlserver.rb — one more assertion in the
  query_requires_identity_insert? test, using the generated MERGE with the
  table hint removed so the aliased target is exercised.

Without the change these fail with 3 failures and 2 errors; with it the file is
green.

schema_test_sqlserver.rb     32 runs, 42 assertions, 0 failures, 0 errors
adapter_test_sqlserver.rb    83 runs, 187 assertions, 0 failures, 0 errors, 1 skip
insert_all_test_sqlserver.rb  1 runs,  1 assertions, 0 failures, 0 errors

bundle exec standardrb is clean for the whole repository.

Branches

The same pattern is present on main, 8-1-stable and 8-0-stable. This PR
targets main. Happy to open backport PRs for the stable branches if you would
like them.

[aidanharan]

@Alt70155 Could you create a PR against the 8-1-stable branch instead of main? The tests aren't passing on main at the moment so cannot tell if your changes introduce any regressions. Thanks

[Alt70155]

Thanks for taking a look. Opened #1405 against 8-1-stable.