chore: remove baseapp from x/accounts

[aljo242]

Description

part of #22903

• Moves integration test MsgRouter logic to coretesting so that it can be used as a common function in keeper and integration level tests.
• Remove baseapp import entirely in `x/accounts

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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• updated the relevant documentation or specification, including comments for documenting Go code
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Summary by CodeRabbit

• Dependency Updates

  • Updated google.golang.org/grpc to version v1.69.4 across multiple modules
  • Added cosmossdk.io/core/testing replacement directives in various go.mod files

• Testing Infrastructure

  • Introduced new msgrouter package for message routing in integration tests
  • Replaced integration.RouterService() with msgrouter.RouterService() across test suites
  • Updated query client and router service implementations

• Module Refactoring

  • Simplified codec and query handling in test environments
  • Restructured module replacements in go.mod files
  • Updated Go version in core/testing module to 1.23.1

• Code Cleanup

  • Removed deprecated router and event service implementations
  • Simplified message and query handler registrations

[julienrbrt]

core/testing is getting great 🚀