spike: explore dbt Core 1.12 v2 parser support (fixes #770)

[axellpadilla]

Summary

Investigation and infrastructure for dbt Core 1.12 --use-v2-parser support with the dbt-sqlserver adapter. Closes #770.

Finding

The v2 parser (dbt-core-experimental-parser 2.0.0-alpha.5 / dbt Fusion) does NOT recognize the sqlserver adapter type. Its profiles.yml validation rejects type: sqlserver as an unknown variant. Until sqlserver is added to the v2 parser's adapter registry, --use-v2-parser cannot be used with dbt-sqlserver.

What's included

Area	What
Dependencies	dbt-core 1.12.0 + dbt-core-experimental-parser>=2.0.0a5 (dev dep)
CI	New v2-parser-tests job in integration workflow. Blocking, push/PR only.
Tests	13 test scenarios: parse, compile, build, docs generate, incremental, indexes, column types -- all marked xfail(strict=True) pending upstream support
Tooling	devops/scripts/compare_manifests.py for v1 vs v2 manifest diffs
Docs	docs/contributing/v2-parser.md -- status, references to source code, local setup
Make	make v2-parser-test target

What happens when upstream adds sqlserver

All 13 xfail markers are strict=True. The moment dbt-core-experimental-parser adds sqlserver to its adapter enum, the test suite flips from expected-fail to expected-pass -- no code changes needed. Remove the xfail decorators and it's done.

Test output

tests/functional/adapter/v2_parser/test_v2_parser_basic.py::test_parse_with_v2_parser XFAIL
tests/functional/adapter/v2_parser/test_v2_parser_basic.py::test_compile_with_v2_parser XFAIL
...
13 xfailed in 1.01s

References

• dbt-core 1.12 release notes: --use-v2-parser flag (#13029)
• dbt/cli/params.py lines 805-818: flag definition, default binary path
• dbt/parser/fusion.py: handoff logic spawning v2 parser subprocess