fix: preflight schemas apply correctly when chained before for

[PranavAgarkar07]

closes #15746

Root cause

preflight() and for() live in different closure scopes. preflight() is defined inside create_instance() and stores the preflight schema in that closure. for() is defined on the outer instance after create_instance() returns — when called, it creates/reuses a different cached create_instance(key) instance whose closure has preflight_schema = undefined.

This means createPost.preflight(schema).for("some id") silently drops the preflight schema, while createPost.for("some id").preflight(schema) works correctly because both methods operate on the same instance.

Fix

Used a WeakMap in the form() scope to track preflight schemas per instance. Both preflight() (inside create_instance) and for() (on the outer instance) share access to this map:

• preflight() stores the schema keyed by the current instance
• for() checks if the calling instance has a preflight schema in the map and propagates it to the new instance via entry.instance.preflight(schema)

Test

Added a new E2E test page at /remote/form/preflight-for that validates preflight().for('id') works end-to-end.

Verification

• pnpm check — passes (0 errors)
• pnpm -F @sveltejs/kit test:unit — 537/537 pass
• pnpm prepublishOnly — types generate cleanly
• pnpm run lint — linting clean

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[teemingc]

I think this PR may accidentally have pulled in changes from #15859 which we should be separate