[query] remove branchingFactor from HailContext

[ehigham]

Change Description

Refactored the branching factor parameter to be controlled through feature flags instead of being passed directly to the HailContext constructor. This change:

1. Removes the branching factor parameter from HailContext constructor and methods
2. Adds a new feature flag "branch_factor" with a default value of 50
3. Creates a new agg package with utility functions to retrieve the branch factor from context
4. Updates all code that previously accessed HailContext.get.branchingFactor to use agg.branchFactor(ctx) instead
5. Modifies Python backends to pass branching factor through flags rather than constructor parameters

This approach provides a more consistent way to configure the branching factor through environment variables or programmatic flags, aligning with how other Hail parameters are managed.

Security Assessment

This change cannot impact the Hail Batch instance as deployed by Broad Institute in GCP

[ehigham]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [query] remove unnecessary references to HailContext #14964
• [query] remove branchingFactor from HailContext #14963 👈 (View in Graphite)
• [query] Configure Optimiser via Flags, not HailContext #14952
• [query] Lowering + Optimisation with implict timing context #14731 : 2 other dependent PRs (#14732 , #14748 )
• [query] Lift backend state into (Batch|Py4J)QueryDriver #14698
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