[SunSi12138]

Closes #723

Summary

• cache IRpcSizedCodec<T> / CanExactSize once when each client stream starts
• pass the cached exact-size capability into client stream publication so TryGetEncodedSize remains per-item
• cache the same capability once for generated server outbound streams
• preserve existing exact-size credit-before-serialize and unsized pre-credit budget paths
• add regression coverage that CanExactSize is read once per stream while sizing/serialization remain per-item

Validation

• existing client deadline/publication behavior remains on the same send helpers
• existing generated-server deadline/cancellation flow remains unchanged
• added exact-size generated outbound coverage and non-exact client-stream coverage

Base: dev