Add snapshot unique_key hint for duplicate row errors

[desusaiteja]

Resolves #10864

Problem

When a snapshot unique_key is not actually unique, some adapters return a generic Duplicate row detected during DML action error. The existing message does not point users toward the likely root cause, which makes a straightforward snapshot configuration issue harder to diagnose.

Solution

Append a snapshot-specific hint when snapshot execution surfaces the duplicate-row DML error. The new hint points users to the likely problem: the configured unique_key is not unique in the snapshot query.

This PR adds:

• A small SnapshotRunner.handle_exception override that appends the hint only for the duplicate-row DML error message.
• Unit coverage for the string helper behavior.
• A mocked runner-level test that simulates the adapter-specific error path and verifies the hint is added through SnapshotRunner.handle_exception.

I also ran the change in development and verified the local Postgres path. Postgres did not naturally reproduce this adapter-specific error, so the adapter-specific branch is covered via the mocked runner test.

Checklist

• 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 or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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