feat(chain-sources): fetch headers at caller-requested heights (#2200)

[ANAMASGARD]

Closes #2200

Description

Adds fetch_headers_at_heights(local_tip, heights) -> CheckPoint<Header> to bdk_bitcoind_rpc, bdk_electrum, and bdk_esplora.

Wallets can fetch missing block headers on demand (for MTP, relative timelocks, confirmation timestamps) without paying for prev-11 fetches on every sync. The returned update is reorg-aware and applies via LocalChain::apply_update.

API contract:

• Skips heights already in local_tip
• No-op when all requested heights are known and no reorg → returns local_tip unchanged (empty ChangeSet)
• Update may include bridge/reorg blocks beyond requested heights; no Esplora tip-suffix padding

Shared helpers in bdk_core: bridge_heights, build_fetch_headers_update

Notes to the reviewers

• bdk_bitcoind_rpc: standalone fetch_headers_at_heights (not on Emitter::next_block)
• bdk_electrum: BdkElectrumClient::fetch_headers_at_heights with batch_block_header
• bdk_esplora: new headers_at_heights module; blocking + async trait methods
• Esplora intentionally omits the ~10-block tip suffix used in normal sync/full_scan

Local verification : VERIFIED — all tests passed

cargo test -p bdk_bitcoind_rpc fetch_headers_at_heights
cargo test -p bdk_electrum fetch_headers_at_heights
cargo test -p bdk_esplora fetch_headers_at_heights
cargo clippy -p bdk_core -p bdk_bitcoind_rpc -p bdk_electrum -p bdk_esplora --all-targets -- -D warnings

Changelog notice

• Added fetch_headers_at_heights to bdk_bitcoind_rpc, bdk_electrum, and bdk_esplora for on-demand CheckPoint<Header> fetches at caller-requested heights
• Added bridge_heights and build_fetch_headers_update helpers in bdk_core

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature

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