fix(start-sdk): drop undefined keys an onWrite transformer produces

[helix-a]

What changes

A file model with an onWrite transformer drops the keys that transformer leaves undefined, instead of writing them out as the word undefined.

filterUndefined now runs exactly once per write, on whatever reaches the format's toFile:

• In the format helpers (rawTransformed), it runs after onWrite, or on the data itself when there is no transformer.
• In FileHelper.raw, it runs on the data, just before the caller's writer.

The filter is removed from serialize(), which no longer exists: writeFile and merge call writeData directly. rawTransformed builds its FileHelperImpl directly, so the filter is not wrapped twice.

Why

Before #3874, the INI writer filtered in exactly this spot: INI.stringify(filterUndefined(inData)), after any transformer. #3874 moved the filter into serialize() so that the env writer got it too. But serialize() runs before the transformer, so a key the transformer itself sets to undefined reached INI.stringify and was rendered as the word.

Found while testing Bitcoin on SDK 3.0, on a VM built from the #4062 head. Bitcoin's bitcoin.conf model maps its form onto every conf key in onWrite (formToFile). On its first write after an upgrade it wrote 17 such lines, including:

rpcauth=undefined
externalip=undefined
connect=undefined
onlynet=undefined

The next read turned connect=undefined into connect-only to a peer named "undefined", which fails that field's address pattern, and init aborted:

init failed ZodError: [{"code":"custom","path":["connectpeer","value","peers",0], "message":"Must be either a domain name, or an IPv4 or IPv6 address. …"}]

The same round trip on SDK 2.0.9 writes none of those lines.

onWrite now receives the data as given, undefined keys included, as it did before #3874. Bitcoin's formToFile/stringifyPrimitives pass an undefined value through unchanged, so the filter after them drops it.

Changelog

No entry. #3874's entry already describes the intended behaviour under the unreleased 3.0.0, and the INI regression never shipped.

Verification

• Test: env, ini, yaml and toml each write through an onWrite that sets a key to undefined. Without the fix, the env and ini cases fail.
• Test: FileHelper.raw hands its writer no key merged as undefined.
• start-sdk make check and the full jest suite (136 tests) pass. Prettier 3.8.3 passes on both files.