Fix member_months extension passthrough double-strip (#1347)

[gopidaxhealthcaresolutions]

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Closes #1347

What happens

With passthrough.strip: true and an extension column on eligibility (e.g. ext_cell_phone), the member_months models fail to compile:

invalid identifier A.EXT_CELL_PHONE

Why

The prefix gets stripped twice. select_extension_columns() always discovers column names by introspecting the original input relation (e.g. normalized__eligibility), so it keeps asking for ext_cell_phone. But an upstream CTE has already renamed that column to cell_phone via the first (stripping) call, so the second call references a column that no longer exists.

Fix

This follows the convention already used by every other intermediate model in the repo (e.g. int_eligibility_casting, core__int_patient_casting, the core__stg_claims_* models): intermediate models pass strip_prefix=false so the prefix is preserved end-to-end (and downstream introspection can still identify the extension columns by prefix), and only the final model strips.

The prefix should be stripped exactly once, at the final output (core__member_months, which already calls the macro with the default strip behavior). This PR adds strip_prefix=false to the four intermediate call sites that were missing it:

• models/core/staging/core__int_member_months.sql (2 calls: stg_eligibility, joined)
• models/core/staging/core__stg_claims_member_months.sql (2 calls)

Behavior impact

• Default config (strip: false): no change — extension columns keep their prefix through the whole chain, exactly as before.
• strip: true: previously a hard compile failure; now compiles and the prefix is stripped once in the final core__member_months output.

Testing

Reproduced both the failure and the fix locally on DuckDB with a focused project mirroring the exact double-call CTE chain (int → stg → final) and an ext_-prefixed eligibility column:

• Before (strip: true): Binder Error: Values list "a" does not have a column named "ext_cell_phone" — matches the reported bug.
• After (strip: true): chain compiles and runs; final output column is cell_phone (stripped once), data intact.
• After (strip: false, default): final output column is ext_cell_phone (preserved) — confirms no regression for existing users.

The existing extension_column_unit_tests.yml unit tests for this chain also pass.
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