Answer rate-limit denials and expose the budget

[sergeyfast]

The rate limiter answered every denial with a text/plain 429 and
Retry-After: 10, which an MCP bridge reports to the model as "server
unavailable": the model could neither wait sensibly nor ask for less. This
makes a denial something a client can read and act on, and makes the hourly
budget visible to handlers and to Prometheus.

Rate limiting

• A denied call with an id gets a 429 carrying JSON-RPC error -32010
  (mcp.CodeRateLimited). data holds reason and retry_after, plus
  budget_used, budget_limit and window when the budget is on. A body
  with no id to answer keeps the plain 429; a refusal reads at most 64 KiB
  of the body to find the id.
• Retry-After is the real wait: until the budget window rolls, until the
  next RPM token, one second for a concurrency slot.
• Config.Exempt(method, name) spares cheap calls — a help tool, a static
  resource — the hourly budget. They still pay the rate and concurrency
  limits.
• Remaining(ctx) and Budget let a handler see what is left and narrow its
  work before it runs into the ceiling.
• ChargeFor(ctx, label, work) with two new metrics:
  app_mcp_ratelimit_charge_seconds{label} (a histogram that adds up to the
  budget) and app_mcp_ratelimit_budget_used_seconds{user}.
• Fix a slow request refunding its own RPM tokens.

Transport

• 400 repeated_key: a request that names a member twice in the envelope,
  params or _meta, in any case. fastjson reads the first occurrence by
  exact bytes, while encoding/json (which decodes the call for zenrpc) reads
  the last and ignores case — so Mcp-Name, Mcp-Method and Exempt could
  be checked against one value while another ran.
• 400 missing_id: a method other than notifications/* sent without an id
  or with a null one. zenrpc ran such a call detached from the request,
  outside the limiter's concurrency slot and budget. MCP requires a non-null
  id on every request, and Streamable HTTP answers input the server cannot
  accept with an HTTP error status.

Behavior changes

• A 429 for a call with an id is JSON rather than text, and Retry-After is
  no longer a constant 10.
• A request that repeats a member, case-insensitively, is a 400.
• ping, initialize or any other non-notifications/* method without an
  id, or with "id": null, is a 400 instead of a 202.
• app_mcp_transport_rejected_total gains repeated_key and missing_id.

Testing

make test, make test-race and golangci-lint v2.13.2 are clean. New tests
cover each refusal end to end through the transport in both eras, the
case-folded repeated key against the real dispatcher, a body with 100k
members staying linear, and the budget window and gauge accounting.