[fix](fe) Support precreated streaming job targets

[JNSimba]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Streaming Job converted every source JDBC column and generated target DDL before checking whether the mapped Doris target table already existed. An unsupported source type therefore prevented CREATE JOB even when the user had already created a compatible target table. This change resolves the mapped target table first and skips source-to-Doris type conversion only when that target already exists. Source table identity, primary-key validation, excluded-primary-key validation, and automatic table creation for missing targets remain unchanged.

Release note

Allow FROM source TO DATABASE streaming jobs to use precreated target tables without requiring FE automatic type mapping.

Check List (For Author)

• Test: Regression test added but not run as requested; expected output was derived from the deterministic query result
• Behavior changed: Yes. Precreated targets bypass automatic source type conversion; missing targets preserve the existing behavior
• Does this need documentation: No

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?