Enable full figspec generation for extensions with metadata capability

[JeffreyCA]

Update the figspec (Fig autocomplete spec) generation in azd so that it generates the full command specification (including all subcommands, flags, and argument structure) for extensions that declare the metadata capability. This leverages the foundation merged in #6496 and should build atop latest tracing/metadata improvements such as #6528.

Requirements:

• For each installed extension with the metadata capability, auto-include its full command tree in the figspec output, not just its basic entry point.
• Ensure that generated specs reflect all subcommands and flags per extension metadata.
• Provide test(s) confirming that updates/extensions with metadata are reflected in exported specs without manual intervention.
• (Bonus) Surface recently added extensions azure.ai.agents and azure.ai.finetune in generated completions if they implement metadata.

Rationale:

• Enables improved VS Code IntelliSense/completions for extension commands, aligning with the move toward richer extension metadata (#6496).
• Related: #6541 - azure.ai.* extensions adopting metadata capability.