Centralise structlog initialisation in BaseDagBundle

[dabla]

BaseDagBundle now owns the structlog logger via a lazy _log property, so subclasses no longer need to construct and bind it themselves in their constructors. I saw all Dag bundles extending this class re-implemented the same logic regarding the structlog initialization when reviewing PR #67016, hence why I created this PR to make the code more DRY.

What changed

• BaseDagBundle gains a _log lazy property that creates a structlog
  logger on first access, bound with bundle_name, version, and
  whatever _log_context() returns.
• A _log_context(self) -> dict template-method hook (returns {} by
  default) lets each subclass declare its own extra log fields without
  duplicating the boilerplate structlog.get_logger(...).bind(...) call.
• GCSDagBundle, S3DagBundle, and GitDagBundle each implement
  _log_context() and drop their manual self._log = log.bind(...) init
  blocks and the now-unused module-level / local structlog.get_logger
  calls.
• LocalDagBundle needs no change — the default _log_context() returns
  {}, which is the correct behaviour for a bundle with no extra context.

Why lazy?

BaseDagBundle.__init__ is called via super().__init__() before subclass-specific attributes (e.g. bucket_name, bare_repo_path) are set. Evaluating _log_context() at base-__init__ time would therefore fail or produce incomplete context. The lazy property defers evaluation until first use, by which point all __init__ work across the hierarchy is complete.

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Claude Sonnet 4.6

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