V5 tracker: Retire MockOptional in favor of explicit optional-parameter controls

[cwinland]

Docs reference: docs/roadmap/README.md

This tracker retires MockOptional in favor of explicit optional-parameter controls once the migration path is complete.

Acceptance checklist

• Remaining MockOptional examples and guidance are rewritten toward the explicit options model before removal begins.
• Any still-missing helper exposure for explicit optional-parameter controls is identified before the alias is removed.
• Analyzer, migration-doc, and release-note dependencies are explicit before implementation begins.
• The removal or obsoletion path for MockOptional is explicit enough that existing users can tell what to do instead.
• Follow-on implementation can remove the alias without leaving optional-parameter guidance ambiguous.

Follow-on boundaries

• broader compatibility cleanup stays in #141
• this issue does not reopen provider-native optional-parameter setup semantics