Bug: Context getters have panics, silent overwrites, and inconsistent behavior

## Summary

The `rpcserver` context helpers have two concrete issues:
1. Two functions panic on missing values
2. Inconsistent error-handling across getters

---

## Bug 1: Panics

```go
// jsonrpc_server.go:432 - PANICS if sizeKey not in context
func GetRequestSize(ctx context.Context) int {
return ctx.Value(sizeKey{}).(int)
}

// jsonrpc_server.go:436 - PANICS if urlKey not in context
func GetURL(ctx context.Context) *url.URL {
return ctx.Value(urlKey{}).(*url.URL)
}
```

Call these outside a handler (e.g., in a test, a goroutine, or refactored code path) and they crash.

---

## Bug 2: Inconsistent Contracts

| Function | Missing Value Behavior |
|----------|----------------------|
| `GetHighPriority` | Returns `false` |
| `GetSigner` | Returns zero address |
| `GetRequest` | Returns `nil` |
| `GetBuilderNetSentAt` | Returns zero time |
| `GetRequestSize` | **Panics** |
| `GetURL` | **Panics** |

Callers can't know which are safe without reading source.

---

## Proposed Fix

Replace `context.Value()` getters with a typed wrapper (like Gin/Echo):

```go
type RPCContext struct {
Request *http.Request // guaranteed non-nil
URL *url.URL // guaranteed non-nil
RequestSize int
Signer common.Address
// ...
}

func (h *Handler) Method(rctx *RPCContext, args Args) (Result, error) {
rctx.Request.Header.Get("X-Foo") // always safe
}
```

No panics, no magic keys, explicit contracts.

---

## Why Now

Hit this adding header logging across services. Every call site needed nil checks or risked panics. The current design makes simple tasks error-prone.

Start in jsonrpc_server.go around lines 432-436 and inspect the existing context getters and their call sites. Compare the missing-value behavior across GetHighPriority, GetSigner, GetRequest, GetBuilderNetSentAt, GetRequestSize, and GetURL, then determine the scope of introducing the proposed RPCContext wrapper and verify that missing values no longer panic.

Written by the indexing model from the issue text.