Remove the Firehose ExEx

[sduchesneau]

The Firehose ExEx (reth_firehose::run_exex) no longer traced blocks: its trace_block call is commented out and the engine's payload validator does the tracing. It was only kept alive for two startup calls, on_blockchain_init (FIRE INIT) and the genesis block emit, while paying for an ExEx write-ahead log and a notification loop.

This removes it. bin/reth now calls the new reth_firehose::init_blockchain(&provider) from on_component_initialized.

Why that hook and not on_node_started

Tracer::on_block_start panics if on_blockchain_init has not run. In EngineNodeLauncher:

1. with_genesis() writes the genesis block
2. with_components() builds the components and calls on_component_initialized synchronously
3. the engine is built, the engine API server starts, the consensus engine task is spawned (and may start backfill sync right away)
4. on_node_started is called

The ExEx ran on_blockchain_init in a spawned task, with no ordering guarantee against step 3. A node-started hook would be worse. The component-initialized hook runs before the engine exists, and the genesis block is already in the database by then.

API changes in reth-firehose

• Removed: run_exex, emit_genesis_block_if_empty (took an ExExContext), the ExEx-only runner::trace_block, and the reth-exex, reth-ethereum-forks and futures dependencies. The runner module is now init.
• Added: init_blockchain(&provider), which emits FIRE INIT and then the genesis block on an empty chain.
• Kept: emit_genesis_block_on_empty_chain, for embedders that report their own name or version in FIRE INIT.

FIRE INIT output is unchanged (reth <reth-firehose crate version>).

Downstream

Nothing breaks until this is merged into the other release branches, since each consumer pins a tag from its own branch. When it is:

• reth-bsc calls reth_firehose::run_exex (src/main.rs) and must switch to init_blockchain.
• Base stops using it in Get rid of all ExEx hooks for firehose base#32.
• op-reth and world-chain only use emit_genesis_block_on_empty_chain, which stays.

Testing

• cargo test -p reth-firehose -p reth-firehose-tests passes.
• Battlefield reth-dev (Amsterdam, battlefield main at 0e0c41f): 93 passing, 0 failing. fireeth reads FIRE INIT 3.1 reth 2.7.0.
• cargo clippy -D warnings and cargo doc -D warnings on reth-firehose fail on lines in executor.rs that this PR does not touch.