Remove parsed-AST cache from bulk metric derivation

[shangyian]

Summary

After the frozen measures storage change in #2050, deployments could cause errors like:

dj                 | Traceback (most recent call last):
dj                 |   File "/code/datajunction_server/api/deployments.py", line 199, in _run_deployment
dj                 |     execute_result = await deploy(
dj                 |   File "/code/datajunction_server/internal/deployment/deployment.py", line 32, in deploy
dj                 |     return await orchestrator.execute()
dj                 |   File "/code/datajunction_server/internal/deployment/orchestrator.py", line 378, in execute
dj                 |     downstream = await self._execute_deployment_plan(deployment_plan)
dj                 |   File "/code/datajunction_server/internal/deployment/orchestrator.py", line 1206, in _execute_deployment_plan
dj                 |     derived = await self._derive_measures_for_deployed_metrics()
dj                 |   File "/code/datajunction_server/internal/deployment/orchestrator.py", line 1301, in _derive_measures_for_deployed_metrics
dj                 |     await derive_frozen_measures_bulk(self.session, revision_ids)
dj                 |   File "/code/datajunction_server/internal/nodes.py", line 816, in derive_frozen_measures_bulk
dj                 |     measures, derived_sql = await extractor.extract(
dj                 |   File "/code/datajunction_server/sql/decompose.py", line 829, in extract
dj                 |     base_components, derived_ast = self._extract_base(base_ast)
dj                 |   File "/code/datajunction_server/sql/decompose.py", line 993, in _extract_base
dj                 |     func.parent.replace(from_=func, to=result.combiner)  # type: ignore
dj                 | AttributeError: 'NoneType' object has no attribute 'replace'

This is because derive_frozen_measures_bulk was passing a shared query cache into MetricComponentExtractor.extract, which returned deepcopy views of cached query ASTs. Because Function.__deepcopy__ returns self, Function nodes ended up shared across copies with stale parent back-pointers, and _extract_base crashed at func.parent.replace(from_=func, to=...) during any deployment that touched metrics.

The fix drops the cache entirely: extract now always calls parse(query) fresh, which yields an AST with intact parent pointers. Re-parsing per metric is cheap relative to the DB work the bulk path already avoids.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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