Validate outpost client startup configuration

[huangminghuang]

Summary

• require every Ethereum and Solana outpost client signer reference to match an explicitly named --signature-provider
• enforce the correct target chain and native key type for each outpost
• validate explicit Ethereum chain IDs with eth_chainId during startup using checked uint32 parsing
• validate every Solana endpoint with getGenesisHash, and compare an optional configured expected genesis hash
• bound client construction, DNS, and startup RPC validation to five seconds
• add deterministic loopback RPC fixtures, startup validation coverage, provenance tests, and updated operator documentation

Why

Outpost client configuration previously resolved signers through the general provider registry, so anonymous/default/programmatically created aliases could satisfy a signer reference. Ethereum chain IDs and Solana cluster identities also were not verified against the configured RPC endpoint during startup. A misconfigured node could therefore boot with the wrong signer or remote chain and fail later in relay operation.

This change fails startup with a configuration error before relay work begins. The legacy three-field Ethereum client format remains supported and does not perform an identity RPC when no chain ID is configured. Solana endpoints are always identity-checked as required.

Validation

• release build of test_signature_provider_manager_plugin, test_outpost_ethereum_client_plugin, and test_outpost_solana_client_plugin
• focused CTest suites: 3/3 passed
• json_rpc_client_tests: 10/10 passed
• 100 repeated Ethereum three-field no-RPC compatibility runs
• git diff --check
• independent C++/behavior/test review loops and final Claude review: clean

Notes

This branch is based directly on origin/master; it does not include or depend on PR #516.