chore(release): prep v1.12.3

[mdrakiburrahman]

Why this change is needed

Cuts a new patch release v1.12.3 to ship the seven PRs merged to main since v1.12.2:

• feat(clone): support shallow clone with cross-workspace destinations #207 — dbt clone fixes for fresh targets + cross-workspace destinations
• fix: derive int_tests token expiry from JWT exp instead of hardcoded 2028 timestamp #205 — derive int_tests token expiry from JWT exp (was hardcoded to 2028, bypassing refresh)
• chore: remove dead Thrift exception handler in connections.py #203 — remove dead Thrift exception branch in exception_handler
• refactor: extract parse_retry_after to shared helper and drop datetime.utcnow() #200 — consolidate four _parse_retry_after copies into _http_utils.py and drop deprecated datetime.utcnow()
• fix: stop forcing botocore/boto3 loggers to DEBUG at import time #198 — stop forcing botocore/boto3 loggers to DEBUG at import time
• fix: pass re.DOTALL as keyword arg in Livy comment-stripping regex #195 — pass re.DOTALL as a keyword arg in Livy comment-stripping re.sub (was silently setting count=16)
• fix: stop suppressing exceptions in six __exit__ methods #193 — stop suppressing exceptions in all six __exit__ methods across singleton_livy.py and concurrent_livy.py

How

Release-prep only — no functional code change:

• Bumped src/dbt/adapters/fabricspark/__version__.py from 1.12.2 → 1.12.3 (patch bump — all changes are bug fixes or internal refactors; no new public API)
• Prepended a ## v1.12.3 section to CHANGELOG.md summarizing the seven PRs above (generated via the generate-changelog skill from .temp/VERSION_CHANGELOG_FULL.md)

The existing v1.12.2 section is preserved unchanged below the new entry.

Test

N/A — release metadata only. CI on this branch will exercise the full lint/build/test matrix against the released code paths to confirm the bump is green before tagging.

Per docs/RELEASE_CHECKLIST.md, the next steps after this merges:

1. Confirm CI is ✅ on main
2. Create GitHub Release with tag v1.12.3 against main (triggers release.yml → PyPI publish)
3. Verify dbt-fabricspark==1.12.3 on pypi.org