fix(otlp): lower gRPC max message size cap to 50MB

Align with trace agent's TRACE_REQUEST_BODY_LIMIT (50MB) instead of
64MB. More appropriate for Lambda's memory-constrained environment.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jchrostek-dd

bottlecap/src/otlp/grpc_agent.rs

@@ -23,7 +23,7 @@ use crate::{
 
 const OTLP_AGENT_GRPC_PORT: u16 = 4317;
 const DEFAULT_MAX_RECV_MSG_SIZE: usize = 4 * 1024 * 1024; // 4MB default
-const MAX_RECV_MSG_SIZE_CAP: usize = 64 * 1024 * 1024; // 64MB cap to prevent DoS
+const MAX_RECV_MSG_SIZE_CAP: usize = 50 * 1024 * 1024; // 50MB cap (matches trace agent)
 
 struct OtlpGrpcService {
     config: Arc<Config>,