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Add RunConfig seam for injected middleware configs (#5495)

* Add RunConfig seam for injected middleware configs

The operator's external-auth handlers (e.g. the enterprise OBO handler)
build a complete types.MiddlewareConfig and need a way to hand it to the
RunConfig the proxyrunner reads. Upstream had no such path: the builder
exposed no append-style middleware option, and PopulateMiddlewareConfigs
rebuilds MiddlewareConfigs from typed fields, overwriting anything a
handler set. The runtime factory for these types is already registered,
so the only missing piece is an operator-to-RunConfig carrier.

Add a generic, OBO-agnostic seam in pkg/runner so a handler reached via
*[]RunConfigBuilderOption can inject pre-built configs that survive
population:

- RunConfig.AdditionalMiddlewareConfigs: JSON/YAML-serializable carrier
  for handler-supplied middleware configs.
- WithAdditionalMiddlewareConfigs: builder option with append semantics
  (multiple calls/args additive, nil entries skipped).
- PopulateMiddlewareConfigs splices the carrier into the backend-egress
  group — after auth and header-forward, before recovery — so an
  injected egress middleware has the final say on the outbound request.

Upstream carries the configs verbatim and never inspects their
parameters; the type identity is supplied by the caller. No OBO or
enterprise symbol is introduced. Regenerated the server swagger spec for
the new field.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Document seam constraints and pin idempotency

Addresses stacklok/toolhive#5495 review comments:
- LOW pkg/runner/config_builder.go (3397799561): note the injected configs
  are consumed only by PopulateMiddlewareConfigs (operator path); the CLI
  flag path ignores AdditionalMiddlewareConfigs
- LOW pkg/runner/middleware.go (3397799567): document that the carrier is
  intentionally not cleared — idempotent overwrite plus serialized fallback
- LOW pkg/runner/middleware_test.go ... (continued)

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