runner: add DBConnFunc hook for custom DB connections

[Areson]

Summary

Add an optional DBConnFunc callback to StageRunnerConfig and ArchiveRunnerConfig that lets callers inject a custom *sql.DB (e.g. with Datadog APM tracing) without adding tracing dependencies to the OSS library.

When nil (the default), the existing dbconn.New() path is used, preserving full backward compatibility.

Motivation

Services that embed polt may need to instrument MySQL connections with tracing. Without a hook, the only option is to fork or modify the OSS library to add tracing imports — which would pull tracing dependencies into the OSS project.

This hook keeps the tracing concern in the caller while giving it full control over how *sql.DB instances are created.

Changes

• db_helper.go: Define DBConnFunc type
• stage_runner.go: Add DBConnFunc to config/struct, use it in setupDBAndCheckIfRunSucceeded() for both primary and replica DB connections
• archive_runner.go: Same pattern for archive runner
• db_helper_test.go (new): 10 unit tests covering type assignability, nil-by-default backward compat, storage on both runners, correct DSN/config passthrough, and error propagation

Testing

All new unit tests pass without requiring a live MySQL instance. Full package builds cleanly.