Make union relations case agnostic for include + column_override

[ttusing]

resolves #1017

Problem

See #1017 - passing both include and column_override causes column_override to be case sensitive and silently fail/default to using first relation's datatypes.

AI summary from Claude:

Looking at the code, I can explain why the case sensitivity issue occurs with the include parameter. Here's what's happening in the original code:

1. When columns are processed from each relation, there's a check for inclusion that uses lowercase comparison:
{%- elif include and col.column | lower not in all_includes -%}

2. The column names that pass this check are stored in column_superset using their original case:
{%- do column_superset.update({col.column: col}) -%}

3. Later, when applying the column_override, the lookup is done using the original case of the column name from column_superset, but the column_override dictionary is used as-is without any case conversion.

This creates a mismatch because:

* The include filter converts column names to lowercase for comparison
* But the resulting column_superset maintains the original case of the columns
* Then the column_override lookup fails because it's trying to match the original case against whatever case was provided in the column_override dictionary
* This is why when using include, the column_override needs to match the original case of the columns in the source tables - because the case preservation in column_superset creates a case-sensitive context for the column_override lookup.

Solution

Continuing AI generated output:

Our fix addresses this by converting both the column names and types in column_override to lowercase at the beginning:

{%- set lowercase_column_override = {} -%}
{%- if column_override is not none -%}
    {%- for col, type in column_override.items() -%}
        {%- do lowercase_column_override.update({col | lower: type | lower}) -%}
    {%- endfor -%}
{%- endif -%}

And then using lowercase comparison when looking up the override:

{%- set col_type = lowercase_column_override.get(col.column | lower, col.data_type) %}

This makes the behavior consistent regardless of whether include is used or not. The bug report should emphasize that this is a consistency fix that makes the column_override behavior case-insensitive in all scenarios, matching the case-insensitive behavior that already exists for the include and exclude parameters.

I tested this behavior locally and it works.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• [] This PR includes tests, or tests are not required/relevant for this PR
• [] I have updated the README.md (if applicable)

[ttusing]

Reminds me of https://github.com/dbt-labs/dbt-utils/pull/587/files

[ryramshaka]

Can someone approve I need this over 'id' and 'Id' from source tables 😭