fix: partial/required on tuple sequences (#1480)

[drwpwrs]

Fixes #1480

partial() and required() had no runtime effect on tuple sequences — only named properties were transformed. Type inference already handled tuples correctly via TS native Partial/Required, but the schema-level sequence was unchanged.

Adds optionalize() and require() methods to SequenceNode that convert between prefix/defaultable/optional element kinds. Defaultables get stripped to plain optionals (partial) or prefix (required). For required() this matches existing behavior where object defaults are stripped. For partial() this is necessary because the sequence model requires defaultables to precede optionals, so prefix elements can't become optional without also converting defaultables.

Sequences with postfix elements bail out since optionals+postfix is invalid in the sequence model.

.t assertion for defaultable tuples skipped due to #1160.

• Code is up-to-date with the main branch
• pnpm prChecks passing
• New unit tests for partial/required on tuples (prefix-only, string syntax, defaultables)

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