docs+gate: correct fluid plugins examples + teach the docs-honesty gate about CLI commands

[fas89]

What & why

Fixes two real fluid plugins-related doc inaccuracies and teaches the docs-honesty
gate about CLI-command-example drift (the class it was structurally blind to).

Doc corrections (verified against the real command)

• plug-into-fluid-cli.md: the fluid plugins list example listed 4 providers
  under a (5) header — odps was missing. I ran the real command against a
  latest-main CLI venv: it prints aws / gcp / local / odps / snowflake. Fixed.
• your-first-real-plugin.md: the comment claimed PluginMetadata identity
  "surfaces to fluid plugins list" — it does not. The command reads entry-point
  names + allow/block status only and never loads the plugin. Comment corrected to
  say these fields feed registry/marketplace tooling, not fluid plugins list.

Docs-honesty gate extension (the bonus)

• test_documented_cli_commands_exist: every fluid <command> in a doc code fence
  must be a real CLI subcommand (argparse introspection). Runs only when the
  data-product-forge CLI is importable; skips with a clear reason in CLI-less
  SDK CI ("validate where feasible").
• test_fluid_plugins_output_advertises_no_impossible_columns: feasible without
  the CLI — a documented fluid plugins block cannot advertise VERSION/AUTHOR/
  DESCRIPTION columns the command can't emit. Pins exactly this PR's drift class.

Borrow-before-build

Surveyed clitest / cram / markdown-clitest / cli-unit / mktestdocs (run-the-binary
doc testers) + the pytest argparse-introspection pattern. Adapted introspection;
diverged from clitest/cram — they need the fluid binary present, but this
zero-dependency SDK doesn't (and shouldn't) install the CLI in its CI.

Verification

• Ran the real command (fluid plugins list on a latest-main CLI venv) to
  confirm the corrected output.
• Proved the gate catches drift: injecting fluid totallyfakecmd fails the
  command check; injecting the old VERSION/AUTHOR/DESCRIPTION table fails the output
  guard — both green after revert.
• 156 tests pass (+1 skip in CLI-less CI); ruff + black clean.
• Security review: no findings — test-only, no eval/network/subprocess/file-write;
  build_parser introspection is first-party and runs only when the CLI is installed.