Improve config validation in changelog command

[lcawl]

Per #2298 (comment) "We should make https://github.com/elastic/docs-builder/blob/main/config/products.yml the sole source of truth for allowed products."

Summary of Changes

1. Product validation against products.yml: Updated ChangelogService.CreateChangelog to always validate products from the --products option against config/products.yml via IConfigurationContext.ProductsConfiguration. This runs regardless of whether AvailableProducts is set in changelog.yml.
2. Validation of changelog.yml values: Updated LoadChangelogConfiguration to validate that all values in changelog.yml conform to the defaults in ChangelogConfiguration.cs:
   available_types must be a subset of the default types
   available_subtypes must be a subset of the default subtypes
   available_lifecycles must be a subset of the default lifecycles
   available_products (if specified) must all exist in config/products.yml
3. Error handling: All validation errors prevent file generation by returning false or null, ensuring invalid values cannot be written.

The code compiles successfully and follows existing patterns. Products are normalized (underscores converted to hyphens) for comparison, matching the existing product handling in the codebase.

Error handling

If I provide a configuration file that contains an invalid value, the command returns an error and fails to generate a changelog. For example:

./docs-builder changelog add --pr 138879 --type bug-fix --products "cloud-enterprise 4.0.0, cloud-hosted 2025-11-11" --title "Fixes cached errors during upgrade" --config blah/changelog.yml

Returns the following error when my changelog.yml contains an invalid product value:

The following errors and warnings were found in the documentation

Error: Product 'cocoa' in changelog.yml is not in the list of available products from config/products.yml. Available products: apm, apm-agent, ...
 
Error: Failed to load changelog configuration

Likewise if I don't use a changelog.yml configuration but use a value that's not in the list of global values:

./docs-builder changelog add --pr 138879 --type magic --products "cloud-enterprise 4.0.0" --title "Fixes cached errors during upgrade"

...I get an error and the changelog isn't created:

Error: Type 'magic' is not in the list of available types. Available types: feature, enhancement, bug-fix, known-issue, breaking-change, deprecation, docs, regression, security, other

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

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2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: composer-1 agent