Support explicit CLI input format selection with `--format`

[Seddryck]

Summary

Add an explicit --format option to the CLI input pipeline so callers can select the input format independently of the input file extension.

Initially support:

--format csv
--format json

Motivation

The CLI should normally infer the reader from the input filename, for example:

data.csv  -> CSV reader
data.json -> JSON reader

An explicit format override is still useful when:

• the input file has no extension;
• the extension is misleading or non-standard;
• input is later accepted from stdin or another source without a filename;
• scripts should make the expected format explicit.

Expected behavior

expressif run "<expression>" --input data --format json

uses the JSON reader even though the filename has no .json extension.

Likewise:

expressif run "<expression>" --input data.txt --format csv

uses the CSV reader regardless of the .txt extension.

Resolution rules

1. If --format is specified, it takes precedence over filename-based format detection.
2. If --format is omitted, infer the format from the input file extension.
3. If neither an explicit format nor a recognized extension is available, return a clear CLI error explaining that the input format cannot be determined.
4. Reject unsupported format values through normal CLI option validation.

Scope

• Add --format to commands that consume structured input through the common input pipeline, starting with run.
• Support csv and json values initially.
• Route format selection through the same reader abstraction used by automatic extension detection.
• Add tests for explicit CSV/JSON selection, precedence over extensions, inference fallback, unknown extensions, and invalid format values.

Dependency

JSON support itself is tracked separately in #675.