Improve subcommand validation error messages (#62)

* Add recursive validation for subcommand arguments

Previously, when a subcommand was provided but was missing required
arguments, facet deserialization errors would leak through with unhelpful
messages like 'Error: missing field `name` in type `InitArgs`'.

This commit:
- Adds recursive validation in collect_missing_in_arg_level() that checks
  subcommand arguments when the subcommand is present
- Adds comprehensive test suite (subcommand_errors.rs) with 10 test cases
  covering various missing argument scenarios
- Adds format_missing_cli_args_with_mockup() for user-friendly CLI errors
- Updates driver to use CLI-focused error format when all missing fields
  are CLI arguments

Before:
  Error: missing field `name` in type `InitArgs`

After:
  Error: Missing required arguments

  You provided:
    init

  Missing:
    <name>   # Project name (creates directory with this name)

  Example usage:
    init name

This is still a work in progress - needs refinement for edge cases and
consistency across different argument structures.

* Fix clippy collapsible_match warning

* Accept updated snapshots with new error format

* Refactor CLI error formatting for cleaner empty args handling

- Change format_missing_cli_args_with_mockup() to accept Option<&str>
  instead of requiring comparison against magic string
- Remove 'You provided:' section when no args given
- Show actual program name in examples instead of '<no arguments>'
- Use placeholder syntax in examples: <name> and --flag <flag-name>

Before:
  Error: Error: Missing required arguments
  You provided: <no arguments>
  Example: <no arguments> --flag <value>

After:
  Error: Missing required arguments
  Missing: --flag
  Example: program --flag <flag>

* Implement Ariadne-based visual error format for missing CLI arguments

- Add MissingFieldKind enum to properly distinguish CLI args from config fields
- Show corrected command with missing argument in Ariadne source
- Highlight the missing argument with green color
- Display field documentation directly in the label
- Only use Ariadne format for actual CLI args (not config fields)

Example output:
  Error: missing required argument
     ╭─[ <unknown>:1:6 ]
     │
   1 │ init <name>
     │      ───┬──
     │         ╰──── Project name (creates directory with this name)
  ───╯

Known issues:
- Duplicate Error: prefix (Ariadne + wrapper)
- Source shows <unknown> instead of <cli>

* Fix duplicate Error prefix and source name by using proper architecture

Instead of pre-formatting Ariadne output as a string, we now:
- Build structured Diagnostics with spans on the corrected command
- Set the corrected command as the source (named '<suggestion>')
- Let DriverReport's render_pretty handle the Ariadne formatting

This eliminates both issues:
- No more 'Error: Error:' duplication
- Source shows '<suggestion>' instead of '<unknown>'

The corrected command becomes THE source for the error, which is
architecturally cleaner and reuses existing infrastructure.

* Fix clippy errors: remove loop that never loops and dead code

* Add label field to Diagnostic for separate label text

Main error message is now generic ('missing required argument')
while the label shows field-specific documentation.

Before:
  Error: Project name (creates directory with this name)
     ...
     ╰──── Project name (creates directory with this name)

After:
  Error: missing required argument
     ...
     ╰──── Project name (creates directory with this name)

* Conditionally show help hint only if schema has help field

Now only shows 'Run with --help' if the schema actually defines
a help field (e.g., FigueBuiltins). This prevents suggesting --help
when it's not available.

* Update snapshots

* Include enum variants in help, cf. #60

* Fix one snapshot

* Strip the hexadecimal suffixes that test binaries have so that CI passes reliably.

* Update snapshots

Author: fasterthanlime