refactor: use SDK stream_bundles() for bundle cache pagination

[Evalir]

Closes ENG-2117.

Summary

• Replace manual cursor pagination in BundlePoller with stream_bundles().try_collect() — the SDK now provides BuilderTxCache::stream_bundles() which handles authenticated, paginated bundle fetching. The builder was reimplementing this with a hand-rolled cursor loop.

• Align BundlePoller style with TxPoller — TxPoller already uses stream_transactions().try_collect() and inspect_err for error handling. This PR brings the bundle poller into the same pattern: inspect_err on the result, span guard in the success body, and no enter/drop span dance around the is_closed check.

• Preserve NotOurSlot handling — the special-case trace log for slot-gated bundle fetching is maintained in the inspect_err closure, distinct from the general error counter + warn path.

Net result: -37 lines, +16 lines. Pagination is now fully delegated to the SDK for both transactions and bundles.

Test plan

• make clippy passes with no warnings
• make test passes (8 unit tests, integration tests require env config)
• make fmt clean
• Verify bundle ingestion in staging with >50 bundles available

🤖 Generated with Claude Code

[Evalir]

• refactor: use SDK stream_bundles() for bundle cache pagination #258 👈 (View in Graphite)
• main

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[Fraser999]

Just some tracing issues, some of which exist in the TxPoller too.