refactor(cli): Replace af api flags with subcommands

[kaxil]

Summary

Refactors the af api command to use subcommands instead of flags for listing endpoints and fetching the OpenAPI spec:

• af api --endpoints → af api ls
• af api --endpoints --filter var → af api ls --filter var
• af api --spec → af api spec

Design rationale

Why subcommands instead of flags?
Aligns with CLI conventions where distinct operations are subcommands (like git remote add vs git remote -a). The ls and spec operations are semantically different from making API calls.

Why not use Typer's native subcommand feature?
Typer processes positional arguments before checking subcommands. With formal subcommands registered, af api dags -F limit=10 would fail with "No such command 'dags'". Instead, we detect reserved names (ls, spec) in the main handler and dispatch to internal functions.

Why move OpenAPI fetching to adapters?
The original CLI code had AF2/AF3 fallback logic (try /openapi.json, fall back to /api/v1/openapi.yaml). This version-specific behavior belongs in adapters, following the existing pattern for all other Airflow operations.

Usage

# List all endpoints
af api ls

# Filter endpoints
af api ls --filter variable

# Get OpenAPI spec
af api spec
af api spec -i  # include headers

# Existing functionality unchanged
af api dags
af api dags -F limit=5
af api variables -X POST -F key=test -f value=hello

Gotchas

• ls and spec are reserved endpoint names. If Airflow ever adds actual /ls or /spec endpoints, users would need af api /ls --raw to access them directly.