Add README documentation for lib/common library components

[AdamKing0126]

Summary

• Expands lib/common/README.md to document all components: FileHelpers, HashHelpers, PdfHelpers, S3Helpers, VirusScan, ConvertToPdf, and links to the client/ and models/ subdirectory READMEs
• Adds lib/common/exceptions/README.md with the full exception class hierarchy grouped by category, base class descriptions, and usage examples
• Adds lib/common/models/README.md covering Base, Collection, RedisStore, Resource, CacheAside, and supporting types

Test plan

• Review each README for accuracy against the source files
• Confirm exceptions/README.md class table matches files in lib/common/exceptions/
• Confirm models/README.md examples are consistent with the actual class APIs

[Copilot]

Pull request overview

Expands documentation for the lib/common shared utility library in vets-api, adding comprehensive READMEs for common models and exception hierarchies to make component usage and contracts easier to discover and apply consistently.

Changes:

• Expanded lib/common/README.md to document Common library components and link to sub-READMEs.
• Added lib/common/exceptions/README.md describing the exception hierarchy and usage.
• Added lib/common/models/README.md documenting non-ActiveRecord model base classes, collections, and Redis-backed models.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
lib/common/README.md	Provides an index-style overview of Common components with usage snippets and links.
lib/common/exceptions/README.md	Documents exception classes, intended HTTP statuses, and raising patterns.
lib/common/models/README.md	Documents Common model base classes/concerns (Virtus/Dry::Struct/RedisStore/Collection).

[AdamKing0126]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Applied all review feedback in commit cbc7a43:

• Common::VirusScan: Updated to clarify false means "not confirmed clean" (includes infected and scanning skipped/blocked), and added a note that Settings.clamav.mock should be cast to a real boolean via ActiveModel::Type::Boolean.new.cast(...).
• Common::ConvertToPdf: Updated to clarify it always writes to a ClamAV temp file first; for PDFs it returns a temp-file copy (caller must clean up), not a transparent pass-through.
• Common::Models intro: Reworded to avoid implying a Common::Models::* namespace — classes live directly under Common.
• Comparable::Ascending/Comparable::Descending → corrected to Common::Ascending/Common::Descending.
• Exceptions table: Fixed TimeoutError → Timeout (status 500, not 504); moved FailedDependency (424) to the 4xx section; fixed OpenIdServiceError to %{status}; corrected ServiceError to 500, UpstreamPartialFailure to 502, and PrescriptionRefillResponseMismatch to 502.