Run audits concurrently using concurrent_tasks setting

[airhorns]

Description

Before, audits were run one at a time after each model, and block the execution of downstream models. If you are a heavy user of audits, this trades correctness for a lot of speed, where the run will only move forward if all the audits pass. But, if you want to get the run done and then audit after the fact, it really slows things down. dbt takes a different approach, where you can do a run, and then later run tests. I wouldn't say that's better, but in some circumstances, especially with non-blocking audits or warnings, you don't need to know their state before moving on and building the next model.

So, in an effort to facilitate running audits out of band as fast as possible, or faster when run in band, this changes audits to execute concurrently as part of the DAG! Paired with #5730, this lets folks run the models without any audits, and later run audits with a high degree of paralleism.

Implementation

When audit_only=True, all audit tasks across all snapshots are flattened into a single thread pool instead of following DAG ordering. Since audits are read-only SELECT queries with no side effects, DAG dependencies are irrelevant and all concurrent_tasks worker slots stay filled.

Per-model audit concurrency is also plumbed through SnapshotEvaluator.audit() via a new audit_concurrent_tasks parameter (defaults to sequential). The cross-model path hardcodes this to 1 to avoid nested thread pool multiplication.

The SnapshotEvaluator parameter ddl_concurrent_tasks is renamed to concurrent_tasks to reflect its broader scope.

Test Plan

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO

Closes #5468

[airhorns]

This is a draft PR -- looking for feedback on the general approach and the core implementation -- I am a new contributor so I am very happy to be told this is totally wrong! I used Claude Code to help me with this, but I promise its not a drive-by slop PR, and I'm making use of this in a fork in my own project.