Fix incomplete config schema for guardrails so it can be backported to 1.3.x

[Tracker]
Update Summary: [One-line status update for stakeholders]
Short Description: Fix incomplete config schema for guardrails so it can be backported to 1.3.x
Check-in Date: MM/DD/YYYY
[/Tracker]

Problem/Motivation

The work in #3582856: Add Input Length Limit guardrail plugin against DoW attacks adds schema definitions for the new guardrail configuration, but the underlying problem is broader than the new input_length_limit plugin. The current guardrail config entities already have incomplete schema definitions for guardrail_settings, and the guardrail set schema is also incomplete and inconsistent for fields such as stop_threshold and the guardrail plugin ID mappings.

Because #3582856: Add Input Length Limit guardrail plugin against DoW attacks is a feature issue targeting 1.4.x, keeping the schema fix there makes it harder to move the bug fix to 1.3.x as well. Splitting the schema correction into its own bug issue makes the fix clearer, easier to review, and backportable independently of the new guardrail plugin work.

The proposed schema changes are already visible in the related merge request diff: https://git.drupalcode.org/project/ai/-/merge_requests/1407.diff.

Proposed resolution

• Split the config schema fixes out of #3582856: Add Input Length Limit guardrail plugin against DoW attacks into a dedicated bug fix issue.
• Add schema coverage for guardrail_settings using plugin-specific schema definitions keyed by guardrail plugin ID.
• Add the missing labels and schema entries for the current guardrail set configuration, including stop_threshold and the nested pre/post guardrail mappings.
• Keep the new input_length_limit feature work in #3582856: Add Input Length Limit guardrail plugin against DoW attacks, but move the generic schema correction into this issue so it can be committed to 1.3.x and then merged forward.
• Add or keep test coverage that confirms the affected config entities and plugin configuration can be saved without schema gaps.

AI usage (if applicable)

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