Enhance WarnErrorOptionsV2 to support per-instance targeting with discriminators

[jairus-m]

resolves #358

Description

Adds per-flag targeting in warn_error_options. This allows users to silence (or error on) specific event instances via <event_name>:<flag_name> without affecting all event change warnings. From the motivating example from the issue this means being able to config:

flags:
  require_all_warnings_handled_by_warn_error: true
  warn_error_options:
    silence:
      # - BehaviorChangeEvent <-- before: silences ALL BehaviorChangeEvents
      - BehaviorChangeEvent:use_managed_iceberg # after: silence specific targets
      - BehaviorChangeEvent:use_materialization_v2 # after: silence specific targets

Notes on implementation:

• Any event class can support per-instance targeting by overriding/implementing discriminator() (defined on BaseEvent) if more specific targeting is useful for that event class
  • This means WarnErrorOptionsV2 can support any ClassName:discriminator in the future
  • For this PR, BehaviorChangeEvent implements discriminator() to return flag_name
  • No other event classes in this repo really benefit from this, but the generic infra is there
• Existing errors / silenced logic applies the same hierarchies:
  • discriminated > class > Deprecations > all
  • silence > warn > error
• BehaviorChangeEvent still targets all behavior-change events (backwards compatible)

Local Testing n Validation

I have a dbt-core sandbox locally that uses the dbt-databricks adapter. I overrode the dbt-commons dependency in the uv virtual environment and pointed it to my local dev branch and ran manual tests:

1. Control: No silencing, getting both expected warnings (use_managed_iceberg & use_materialization_v2)

2. Test: Silencing `BehaviorChangeEvent:use_managed_iceberg` only 3. Test: Silencing both `BehaviorChangeEvent:use_managed_iceberg` and `BehaviorChangeEvent:use_materialization_v2`

Both tests show and confirm expected behavior!

Related user reports:

• dbt-core #12268
• dbt-core #11471
• dbt-databricks #1089
• dbt-databricks #1266

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

NOTE: This PR was created with AI-assistance (Cursor)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.57%. Comparing base (f21aa0d) to head (a523985).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
dbt_common/helper_types.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
+ Coverage   72.39%   72.57%   +0.17%     
==========================================
  Files          53       53              
  Lines        3746     3770      +24     
==========================================
+ Hits         2712     2736      +24     
  Misses       1034     1034              

Flag	Coverage Δ
unit	72.57% <96.77%> (+0.17%)	⬆️

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