Which issue does this PR close?

• Closes Remove From<String> for Column #17375

Rationale for this change

As mentioned in the issue, the From trait implementations for Column were misleading - they invoked Column::from_qualified_name() which parses and lower-cases field names, making conversions like field.name().into() behave unexpectedly. Requiring explicit calls to Column::from_qualified_name() makes the behavior clear and prevents misuse.

What changes are included in this PR?

• Removed From, From<&str>, and From<&String> trait implementations for Column
• Updated col() and unnest() functions to accept strings directly
• Replaced all .into() usages with explicit Column::from_qualified_name() calls across the codebase

Are these changes tested?

Yes, all existing tests have been updated and pass with the new API.

Are there any user-facing changes?

Yes, this is a breaking API change:

• Users must now call Column::from_qualified_name() explicitly instead of using .into()
• The helper functions col() and unnest() still accept strings directly