Fix Unicode prefix matching in the Dag list

[ehddnr301]

Searching the Dag list for 카페 can omit 카페매출집계 on PostgreSQL with libc en_US.utf8 collation. The code-point-derived upper bound does not follow the database's collation order.

Use substring equality for non-ASCII display-name prefixes. ASCII prefixes and other prefix filters retain their existing behavior. Update the API description to document the exception.

Before / After

Searching for 카페 returns no results before the fix and correctly returns 카페매출집계 after the fix.

AS-IS	TO-BE

Captured from the local UI using the same synthetic data and PostgreSQL en_US.utf8 collation. The before capture uses the original range matcher; the after capture uses substring equality.

In a warm-cache benchmark with 20,000 synthetic Dags, the sum of median count and list query times increased by approximately 3–5 ms. This avoids database-specific collation handling.

Validation:

• Core API/Always and common.compat/FAB tests, with and without DB access.
• PostgreSQL 16 regression and API cases: 9 passed.
• UI tests: 1,865 passed; SimpleAuth UI: 1 passed.
• Generated Python API client tests: 360 passed.
• Pre-commit/manual checks and affected documentation builds passed.

The remaining shared prefix matcher work is tracked separately.

related: #74551

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Was generative AI tooling used to co-author this PR?

• Yes — Codex (GPT-6); Claude assisted with the initial investigation.

Generated-by: Codex (GPT-6) following the guidelines

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Drafted-by: Codex (GPT-6)

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