feat: attach Diagnostic to duplicate table name errors

[haochunchang]

Which issue does this PR close?

• Closes Attach Diagnostic to "duplicate table name" error #14436.

Continues #20720 by @buraksenn, rebased onto current main.

Rationale for this change

If a query uses the same name twice in one FROM clause, DataFusion rejects it, but the error is unhelpful in two ways:

1. No location. The error doesn't point at where in the SQL the repeated name appears.
2. For repeated table names or aliases, DataFusion only notices after it merges the columns, so the error blames a duplicate column instead of saying the table or alias was used twice.

What changes are included in this PR?

1. Duplicate CTE names: attach a Diagnostic at the repeated name, with a note at the first definition.
2. Duplicate relation names in a FROM scope (comma-separated entries, JOINs, parenthesized joins):
   • Each one adds a non-fatal warning Diagnostic. There is exactly 1 warning per duplicate.
   • If planning fails with DuplicateQualifiedField for that name, the error gets a Diagnostic pointing at both relations. The error message text is unchanged.
   • Whether a query plans or fails is unchanged.
3. Names are compared as TableReferences that match the planned qualifiers.
   • A quoted alias "s.t" does not match s.t, and an unaliased table function foo(..) (qualifier foo()) does not match a table foo. (datafusion/sqllogictest/test_files/join.slt.part)
   • Names the check can't resolve are skipped.

What is the testing strategy for this PR?

• datafusion/sql/tests/cases/diagnostic.rs:
  • test_duplicate_cte_name: error Diagnostic spans for duplicate CTEs.
  • test_duplicate_table_name: for each FROM shape, checks that there is 1 warning with the right spans:
    • Queries with overlapping columns fail with the original message plus an error Diagnostic.
    • Queries with disjoint columns and LEFT SEMI joins plan successfully.
  • test_duplicate_table_name_single_warning_in_multi_entry_from: 1 warning, not 2, when a duplicate spans comma-separated entries.
  • test_same_table_name_in_different_scopes: no warning across scopes.
• datafusion/sqllogictest/test_files/join.slt.part: New cases cover a quoted dotted alias vs a schema-qualified table, and a table function vs a same-named table.

Are there any user-facing changes?

• New warning Diagnostics for duplicate relation names. Callers read them with SqlToRel::take_warnings.
• An error Diagnostic is now attached to duplicate CTE and duplicate qualified field errors.
• No breaking changes: the same queries plan or fail as before, with the same error messages.
  • In particular, duplicate aliases that previously planned (e.g. FROM j1 AS t, j2 AS t with disjoint columns) still plan, now with a warning.

🤖 Generated with Claude Code and manually edited by @haochunchang .