feat(loadtest): add --send-rpc-url and --rate-limit-ramp-duration

[praetoriansentry]

Description

Two additions to polycli loadtest:

--send-rpc-url — a secondary RPC endpoint used only to broadcast transactions (eth_sendRawTransaction, or eth_sendRawTransactionPrivate when combined with --private-txs). All other calls — gas estimation, chain ID, nonces, receipts, block polling, and account funding — stay on --rpc-url. This supports:

• Private mempool endpoints that only accept eth_sendRawTransactionPrivate.
• Gossip-only broadcasters (e.g. light clients on the p2p network) that can broadcast transactions but have no chain state to answer other queries.

The send endpoint gets its own HTTP connection pool (sized 2×concurrency, same as the primary) so broadcasts don't contend with read traffic, and falls back to the primary client when the flag is unset. Like --private-txs, it is limited to the modes that broadcast explicitly (transaction, blob, contract-call, recall) and rejected at config validation for the binding-based modes.

--rate-limit-ramp-duration — linearly ramps the rate limit from max(1% of --rate-limit, 1 TPS) up to the full --rate-limit over the given duration (e.g. 3m), then holds steady. Useful for warming up a network gradually instead of starting at full load. Mutually exclusive with --adaptive-rate-limit, which controls the same limiter based on txpool feedback.

No breaking changes; both flags are opt-in and default to current behavior.

Jira / Linear Tickets

• N/A

Testing

• make build, go vet, shadow, golangci-lint (0 issues), go test -race ./loadtest/...
• New config validation unit tests: --send-rpc-url / --private-txs mode allowlist, URL validation, ramp duration bounds and mutual exclusion with adaptive rate limiting
• Live traffic-split verification against anvil with a method-logging proxy as the send endpoint: a 5-tx run produced exactly 5 eth_sendRawTransaction calls on the send endpoint and nothing else; with --private-txs, only eth_sendRawTransactionPrivate. Reads (gas, chain ID, nonces) all went to the primary RPC.
• Live ramp verification against anvil: --rate-limit 300 --rate-limit-ramp-duration 6s starts at 3 TPS and climbs linearly (52 → 102 → 151 → 201 → 250) to 300, then the ramp goroutine exits.
• Regression run without either flag behaves as before; docs regenerated via docutil

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