fix: forward Baozi per-request RPC URL

[AntonsBB]

Summary

• add rpcUrl to the core per-request credential shape
• make RPC-only sidecar requests construct a per-request exchange instead of reusing the cached default
• forward the override into BaoziExchange while preserving the existing private-key fallback
• add a regression test that verifies the Solana connection receives the caller's endpoint

This fixes the factory/sidecar half of the RPC override path. SDK constructor exposure remains tracked separately in #854.

Verification

• npm test --workspace=pmxt-core -- --runInBand test/unit/exchange-factory.test.ts (10 tests passed)
• npm run build --workspace=pmxt-core

Closes #1679