Inline project slug format validation errors

What does this MR do and why?

Adds inline validation to the project slug (path) field during project creation. Instead of showing a single generic format error, users now see specific messages indicating which rule their input violates (start character, allowed characters, end character, forbidden suffix, or minimum length).

References

Closes #597215 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to New project > Create blank project
2. Enter various invalid project slugs and observe the inline error messages

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