feature: Add @RPC scripted test + extend Object filter (#498 Gap 4 Part 2)

[xerial]

Summary

Wraps up #498 Gap 4. The runtime API surface (`@RPC`, `RxRouter`,
`RxRouterProvider`) landed in #501 and the existing `uniHttpClients`
codegen pipeline (`HttpCodeGenerator` + `ServiceScanner`) already
produces the same `POST` routes that an `@RPC`-annotated trait expects.
This PR adds the missing test + polish:

• A second sbt-uni scripted test (`codegen/rpc-client-annotated`) that
  drives codegen against an `@RPC`-annotated trait whose companion
  extends `RxRouterProvider` — the exact migration shape used by
  wvlet-api's `FrontendApi`. Verifies end-to-end that the annotation
  doesn't break the codegen pipeline and that the generated client
  class compiles.
• Extends `ObjectMethodNames` in `RouterMacros` to also exclude
  `scala.Product` members (`canEqual`, `productArity`, `productElement`,
  `productElementName`, `productIterator`, `productPrefix`).
  `Surface.methodsOf` already filters them by owner; this is
  belt-and-suspenders, addressing Gemini's medium-priority follow-up
  on fix: Filter Object methods and prefix StemNode names (#501 follow-up) #502.
• Mirrors the same Product-method exclusion in `ServiceScanner`'s
  `excludedNames` so codegen output stays consistent with router output
  when applied to case-class-flavored services.

Why this closes Gap 4

Issue #498 Gap 4 lists three deliverables:

1. `@RPC` annotation — landed in feature: Add @RPC, RxRouter, and RxRouterProvider (#498 Gap 4 Part 1) #501.
2. `RxRouter` API in `wvlet.uni.http.router` — landed in feature: Add @RPC, RxRouter, and RxRouterProvider (#498 Gap 4 Part 1) #501.
3. `sbt-uni` codegen task equivalent to `airframeHttpClients` —
   already present as `uniHttpClients` (`UniPlugin.scala`), which the
   new scripted test exercises against an `@RPC` trait.

After this PR merges, wvlet-api's `FrontendApi` / `FileApi`-style
`@RPC` trait + `RxRouterProvider` companion shape compiles against
uni and `uniHttpClients` generates the typed RPC client.

Test plan

• `uniJVM/testOnly wvlet.uni.http.router.RxRouterTest` — 10 tests pass
• `sbt plugin scripted test` runs `codegen/rpc-client` (existing
  backward-compat coverage) and `codegen/rpc-client-annotated`
  (new `@RPC` coverage) in the same CI step.

Refs #498 Gap 4 Part 2 (final).

[gemini-code-assist]

Code Review

This pull request introduces a new test case to verify RPC client code generation for traits with companions extending RxRouterProvider. Additionally, it updates ServiceScanner and RouterMacros to exclude standard scala.Product methods from being identified as service routes. Feedback was provided to include productElementNames in the exclusion lists for both files to ensure complete coverage of Product methods.

[gemini-code-assist]

The list of scala.Product methods is missing productElementNames. While productElementName (singular) is included, the plural version productElementNames (which returns an Iterator[String]) should also be excluded to ensure that all standard product-related methods are filtered out during service scanning.

      "canEqual",
      "productArity",
      "productElement",
      "productElementName",
      "productElementNames",
      "productIterator",
      "productPrefix",

[gemini-code-assist]

The list of scala.Product methods is missing productElementNames. For consistency with the intended "belt-and-suspenders" approach to exclude all Product members, please add productElementNames to this set.

    "canEqual",
    "productArity",
    "productElement",
    "productElementName",
    "productElementNames",
    "productIterator",
    "productPrefix"