Fix KSP maven artifacts never being pinned or resolved

[minkuan88]

Problem

KSP processors declared via ksp(...) in Gradle were computed correctly into
WorkspaceDependencies but never made it through the pinning, dependency
resolution, or Bazel rule generation steps. This caused three silent failures:

Bug 1 — ArtifactPinner skipped ksp_maven entirely

shouldRunPinning() and pinArtifacts() only iterated over variantDeps, so
ksp_maven was never checked for staleness and ksp_maven_pin.sh was never
generated.

Bug 2 — DependencyResolutionService missing KSP from maven store

populateMavenStore() only iterated variantDeps, so getMavenDependency()
could not resolve KSP artifacts during Bazel script generation, leading to
unresolvable target references.

Bug 3 — Dangling Bazel target references from dropped KSP processors

RootBazelFileBuilder silently skipped processors with no
SymbolProcessorProvider service entry, but collectKspPluginDeps still
emitted Bazel target references for them, producing targets that don't exist
in the root BUILD.bazel.

Solution

Introduce aggregatedRepos in WorkspaceDependencies — a map of repo-name to
dependency list for artifacts that bypass the per-variant reduction pipeline
(e.g. ksp_maven). This sits alongside the existing variantDeps and is
propagated through all consumers.

Key changes:

• WorkspaceDependencies: Added aggregatedRepos: Map<String, List<ResolvedDependency>>; renamed result → variantDeps (with
  @SerialName("result") to preserve JSON backward compatibility); removed
  kspResult
• ComputeWorkspaceDependencies: Writes KSP deps into
  aggregatedRepos["ksp_maven"] sorted and deduplicated by max version
• ArtifactPinner: Updated shouldRunPinning() and pinArtifacts() to
  iterate both variantDeps and aggregatedRepos
• DependencyResolutionService: Updated populateMavenStore() to also
  populate from aggregatedRepos; added getValidKspProcessorShortIds() which
  returns only processors that have a valid SymbolProcessorProvider service
  entry
• RootBazelFileBuilder: Warns and skips processors with no
  processorClass instead of silently dropping them
• DefaultDependenciesDataSource.collectKspPluginDeps(): Filters KSP
  plugin deps against getValidKspProcessorShortIds() so only processors with
  a generated kt_ksp_plugin target are referenced