Make the proxy request-body size limit configurable

[ChrisJBurns]

Why

#5492 added a request-body size cap (pkg/bodylimit) on the MCP proxies and vMCP, defaulting to bodylimit.DefaultMaxRequestBodySize (8MB) and rejecting oversized bodies with 413. 8MB is a sensible default, but operators should be able to tune it:

• raise it for deployments with unusually large tools/call payloads (e.g. big inline base64 images/documents), or
• lower it for a stricter DoS posture.

This is the planned configurability follow-up to #5492. Non-urgent — the 8MB default covers most real traffic.

Scope

Expose the cap across the three surfaces, mirroring the existing RateLimiting wiring. Zero/unset must fall back to the default (zero never means "unlimited").

• RunConfig.MaxRequestBodySize int64 + WithMaxRequestBodySize builder option (pkg/runner/config_builder.go), read in PopulateMiddlewareConfigs / addBodyLimitMiddleware (0 → DefaultMaxRequestBodySize).
• --max-request-body-size CLI flag (cmd/thv/app/run_flags.go and cmd/thv/app/proxy.go); run task docs.
• Operator CRD field maxRequestBodySize on MCPServerSpec, wired in cmd/thv-operator/controllers/mcpserver_runconfig.go (mirror RateLimiting); run task operator-manifests operator-generate + task crdref-gen.

Notes

• Caps requests only; server-produced response content is unaffected.
• Out of scope: the management API (1MB) and auth-server (64KB) keep their own tighter caps — they don't carry MCP tool payloads.

Refs GHSA-grwg-v9p7-76m2, #5492.