binding-mcp: proxy hydration/forwarding fixes + per-upstream bootstrap credential

[jfallows]

Summary

The remaining MCP proxy hydration/forwarding fixes plus the per-upstream bootstrap
credential — the third and final binding-mcp extraction from the examples/mcp.proxy
branch (#1793), after cache refresh (#1813) and the single session-id contract (#1815).

Changes

• Content-length framing for request bodies — fix: frame client HTTP request bodies
  with Content-Length; feat: frame streamed tools/call / prompts/get /
  resources/read request bodies — the server stamps a guesstimate contentLength on
  McpBeginEx and the client declares a south Content-Length and whitespace-pads (no
  buffering / chunked).
• Grant the proxy reply window when the request reply begins (fixes a north
  tools/list hang from a reply-window race).
• Strip the toolkit prefix from the forwarded tool name in the proxy request body.
• Degrade gracefully when a south route fails cache hydration — skip a never-established
  route (sessionId == null) from hydration and isolate its per-route lifecycle failure
  from the shared hydrater lifecycle, so the healthy routes still aggregate.
• Per-upstream bootstrap credential on the mcp client binding
  (options.authorization.credentials) — used when there is no inbound client token, so the
  cache hydrater and token-less calls can authorize a route that requires it.
• Widen the redirect hash unsigned — RedirectFW carried sessionId.hashCode()
  sign-extended; binding-http folds it via Long.hashCode, so a negative hash routed the
  cross-worker redirect to the wrong worker → re-redirect → HTTP 500 for ~half of sessions
  under multiple workers. Widen with Integer.toUnsignedLong so the fold round-trips.

Notes

General binding-mcp behavior, independent of the example (the example-config edits in two of
these commits are excluded and stay on the example branch). Final binding-mcp extraction;
after this, only the example remains on #1793.