feat(rails): load enabled rail plugins end to end

[Pouyanpi]

Description

Load enabled rail plugins through the complete configuration and runtime path.
Index packaged plugin flow sources by Colang version, include plugin-declared
configuration fields when constructing RailsConfig, and load only flows from
plugins explicitly enabled by configuration.

Stack 3 defines how a catalog can be extended with named plugin entry points;
this PR carries that explicit enablement through configuration construction and
runtime flow loading. Discovery and activation stay separate: installing a
plugin does not make its config fields or flows active by default.

Add an end-to-end example plugin suite covering disabled plugins, enabled
configuration projection, flow loading, configured-surface parameters,
collisions, unknown plugins, cache resets, and both Colang dialects. Static
catalog discovery remains cheap and does not import action modules.

Impact

• An installed rail plugin remains inert until its name is explicitly enabled
  in the application configuration.
• Enabled plugins contribute typed configuration fields and the correct Colang
  flow source to the same paths used by built-in rails.
• Disabled plugins do not alter RailsConfig, load flow files, or import action
  modules.
• Missing plugins, duplicate providers, manifest collisions, and cache resets
  have deterministic, tested behavior.
• Applications that do not enable plugins retain the existing built-in catalog
  and runtime behavior.

Stack

Layer	Branch	Base	PR
Stack 3	pouyanpi/rail-library-stack-3-manifest-contract	develop	#2157
Stack 4	pouyanpi/rail-library-stack-4-builtins	Stack 3	#2181
Stack 4b	pouyanpi/rail-library-stack-4b-flow-gate	Stack 4	#2185
Stack 5	pouyanpi/rail-library-stack-5-lazy-actions	Stack 4b	#2186
Stack 6	pouyanpi/rail-library-stack-6-hardening	Stack 5	#2187

Related Issue(s)

Verification

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Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• I've noted any verification beyond CI and any checks I couldn't run.
• I did not update generated changelog files manually.
• I addressed all CodeRabbit, Greptile, and other review comments, or replied with why no change is needed.
• @mentions of the person or team responsible for reviewing proposed changes.

[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
nemoguardrails/rails/llm/config.py	91.66%	4 Missing ⚠️
nemoguardrails/manifests/catalog.py	92.59%	2 Missing ⚠️

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