fix(recipe): read prompted file parameters

[vonbai]

Summary

• Read file contents for prompted recipe parameters declared with input_type: file.
• Preserve the existing prompt-building flow for other parameter input types.
• Add regression coverage so a file parameter is inserted as contents instead of a literal path string.

Validation

• cargo fmt --check
• cargo test -p goose --no-default-features --features rustls-tls recipe::build_recipe::tests::file_parameter_tests::test_build_recipe_user_prompt_file_parameter_reads_file_content

Note: the focused test run emitted the existing providers test dead-code warning for clear_env; validation still passed.