fix: Preserve inner filter chain when composing RxRouter filters

[xerial]

Summary

• RxRouter.FilterNode.andThen(next: FilterNode) overwrote next.parent, silently dropping intermediate filters when composing already-built filter chains.
• Fix: attach this on top of next's existing parent chain so the composition is associative.

Problem

Given:

RxRouter
  .filter[A]
  .andThen(
    RxRouter
      .filter[B]
      .andThen[C]
      .andThen(routes)
  )

The expected chain is A -> B -> C, but the previous implementation produced A -> C, silently losing B. The bug also surfaced inside StemNode.wrapWithFilter and RxRoute.wrapWithFilter, both of which delegate to FilterNode.andThen(FilterNode) via andThenOpt, so nested StemNodes with their own filter chains were affected too.

Fix

When composing this.andThen(next), walk to the top of next.parent and attach this there instead of overwriting next.parent.

Test plan

• RxRouterTest: existing tests pass
• New test Preserve the outer filter when wrapping a pre-filtered router (simple 2-filter case)
• New test Preserve the full inner filter chain when wrapped by an outer filter fails before the fix, passes after; also asserts equivalence with the flat filter[A].andThen[B].andThen[C].andThen(routes) form
• ./sbt httpJVM/test (Scala 3): 195 tests pass
• ./sbt '++ 2.13' 'httpJVM/testOnly *RxRouterTest *RxRouterConverterTest' passes
• ./sbt netty/test passes (119)
• ./sbt httpCodeGen/test passes (43)

[gemini-code-assist]

Code Review

This pull request fixes a bug in RxRouter where composing filter chains would result in intermediate filters being dropped. The andThen implementation in FilterNode was updated to recursively traverse and attach to the top of the existing parent chain. Additionally, new test cases were added to verify that filter order and completeness are maintained during complex router compositions. I have no feedback to provide.