fix(tycho-indexer): support non-ethereum chains in rpc subcommand

[die-herdplatte]

Problem

tycho-indexer rpc hardcodes the storage gateway chain:

.set_chains(&[Chain::Ethereum]) // TODO: handle multichain

Since the single-chain DB guard landed (0c1249f), starting the standalone
RPC server against any non-ethereum database fails with:

Storage failure: Unexpected storage error: Chain table already contains
'arbitrum' but attempting to initialize with 'ethereum'. Each database
instance must be dedicated to a single chain.

This breaks the protocol-testing range harness for every non-ethereum
protocol: after indexing succeeds, the harness spawns tycho-indexer rpc
for the validation step, which then tries to initialize the freshly synced
DB as ethereum and dies.

Fix

• Add --chain (default: ethereum, so existing invocations are unaffected)
  and --chain-config to the rpc subcommand, mirroring the run subcommand
  — including init_chains so custom chains resolve.
• Use the parsed chain in run_rpc instead of the hardcoded Chain::Ethereum,
  resolving the TODO.
• Pass --chain from the testing harness's start_rpc_server.

The harness is the only invocation of tycho-indexer rpc in this repo (the
other subcommands — index, run, analyze-tokens — were already
chain-aware), so no other call sites needed updating. The dockerized testing
harness builds the binary from the same source tree and picks the fix up
automatically.

Testing

• Validated end-to-end with the range harness on Arbitrum
  (range --package arbitrum-ramses-v3 --chain arbitrum, from feat: add Ramses V3 Arbitrum support #1235):
  indexing, component state validation, on-chain balance checks, 6 simulations
  and 6 executions all pass (1/1). Previously this failed at the RPC-server
  step with the error above.
• rpc without --chain keeps the previous behavior (ethereum default).

Note for infra

If any out-of-repo deployment manifests run tycho-indexer rpc as standalone
RPC replicas against a non-ethereum database, they have been failing on
startup with this same guard error since 0c1249f — after this fix they need
--chain <chain> added, since the default remains ethereum.

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