Function resolve_chart_format 

pub fn resolve_chart_format(
    explicit_format: Option<&str>,
    output_path: Option<&str>,
) -> Result<ChartFormat, McpError>

Resolve the effective output format from an explicit format parameter and/or an output_path’s extension.

Rules:

• Both set: they must agree. Conflict returns InvalidArgument naming both values so the caller can fix one.
• Only format set: parse it via ChartFormat::parse.
• Only output_path set: derive from its extension (.png / .svg). Unknown extensions return InvalidArgument.
• Neither set: default to PNG (matches the pre-change behavior).

The path is only inspected for its extension — the file need not exist.

Errors

• Returns ErrorCode::InvalidArgument if both explicit_format and output_path are set and they disagree on the format.
• Propagates ErrorCode::UnsupportedFormat from ChartFormat::parse for unknown format strings.
• Returns ErrorCode::InvalidArgument (via format_from_extension) when output_path has an extension other than .png or .svg.