feat: replace BundlePoller polling with SSE streaming

[Evalir]

Description

Mirrors #259 (tx-poller SSE) for bundles. Replaces the 1s polling loop in BundlePoller with an SSE subscription to /bundles/feed via BuilderTxCache::subscribe_bundles (newly exposed by the SDK, matched by the tx-pool-webservice endpoint).

The structure matches TxPoller exactly:

• Initial full_fetch paginates through stream_bundles to seed the cache on startup.
• subscribe opens the SSE stream and yields CachedBundles in real time.
• On stream error / stream end, reconnects with exponential backoff (1s → 30s cap), racing the sleep against the block-env watcher so an env change wins and triggers a fresh full refetch immediately.
• On block-env change, the full_fetch runs again to cover anything missed during disconnection.
• NotOurSlot is logged at trace level (expected when the builder is not slot-permissioned) in both the full-fetch and subscribe paths — avoids spurious warn-level noise.

The public check_bundle_cache() wrapper is dropped; the integration test now uses BuilderTxCache::stream_bundles directly, matching the style of tx_poller_test.rs.

Related Issue

Stacked on #259.

Testing

• make fmt passes
• make clippy passes
• make test passes
• cargo doc --no-deps passes with -D warnings
• cargo test --features test-utils --no-run builds all integration tests
• Exercise against a live tx-pool-webservice to verify SSE reconnect and env-driven refetch behavior end-to-end

[Evalir]

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

• feat: replace BundlePoller polling with SSE streaming #269 👈 (View in Graphite)
• feat: replace TxPoller polling with SSE streaming #259
• main

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