fix: add option for new conditional options behavior

[danielcardoso5]

Follow up to the breaking change introduced in #265.

It adds a new disallowNewConditionalOptions(defaults to false) that allows to opt into the breaking change behavior: disallowing new conditional options from being introduced by conditional logic.

This ensures existing usage does not break when #265 is released.

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Note

Medium Risk
Changes how conditional schemas affect field options and validation for consumers who opt in or rely on introducing new options via branches; default behavior is preserved but deprecation warnings may surface in production consoles.

Overview
Adds disallowNewConditionalOptions to createHeadlessForm (default false) so teams can adopt stricter conditional option merging without a breaking release.

With the flag true, mergeSchemaBranch only keeps branch enum / oneOf / anyOf / presentation options that already exist on the base field (narrow or re-label); values the branch adds are dropped. If the base has no option array for that key, branch options can still be applied. With the default false, branches still replace option arrays wholesale (legacy), but a one-time console.warn points callers at the new flag when a branch would add options not on the base.

Conditional rule application now passes full CreateHeadlessFormOptions into mergeSchemaBranch via mutations.ts. MIGRATING.md documents the migration path; tests cover both modes. Unrelated: removes unused convertDiskSizeFromTo from utils.

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[danielcardoso5]

Internal pipeline 🟢

[sandrina-p]

Approved! Just a comment on the warning msg, but besides that it looks great. Thanks for updating the MIGRATING.md file too ^.^