[WIP] feat: add input_file_name() for file-backed scans (plumbing PR)

[ethan-tyler]

Which issue does this PR close?

• Closes Add input_file_name built-in function #6051

Rationale for this change

This is the end to end plumbing PR to get input_file_name() working. Started with an SLT test to define the expected behavior, then built out the plumbing to make it pass. Scoped to SELECT-list only (guaranteed pushdown case) per discussion with @alamb and @adriangb, with broader pushdown support to follow once #19538 lands.

What changes are included in this PR?

Add input_file_name() function that returns the file path for each row by injecting the value at the file opener boundary. Opt in (only when referenced), keeps SELECT * stable, errors on unsupported contexts.

Analyzer rewrite

• Rewrites input_file_name() to reserved column __datafusion_input_file_name
• Annotates TableScan.projected_schema only when needed
• Errors on reserved name collisions

Physical planning + execution

• Planner enables scan time injection when internal field is projected
• FileScanConfig::open wraps opener to append Utf8 column with file location per batch
• Stats/equivalence properties/schema updated for appended field

Optimizer

• OptimizeProjections handles internal column safely (prevents index OOB)
• Regression test: reserved column from source schema not treated as injected

Scope (V1)

• Works in SELECT list only
• Plan time errors for non file sources (VALUES/MemTable), joins (ambiguous file origin), and non SELECT list usage (WHERE/GROUP BY/ORDER BY/HAVING)

Are these changes tested?

Yes.

cargo test -p datafusion-sqllogictest --test sqllogictests -- input_file_name.slt
cargo test -p datafusion-datasource extended_file_columns_inject_input_file_name -q
cargo test -p datafusion-optimizer optimize_projections_keeps_reserved_column_from_source -q

SLT uses CSV for deterministic multi file assertions. Parquet supported via same FileScanConfig path and Parquet specific SLTs can follow.

Are there any user-facing changes?

Yes. New 0-arg volatile scalar function: input_file_name() -> Utf8

CREATE EXTERNAL TABLE t STORED AS PARQUET LOCATION '...';
SELECT col1, input_file_name() FROM t;

SELECT * output unchanged unless input_file_name() is explicitly referenced.

[adriangb]

I think it should be much simpler than this: create a UDF and pass it through ProjectionExprs::transform_exprs(|expr| expr.transform(|expr| // if expr is our ScalarUDF, replace with literal filename))