Dead code in search threshold schemas: unreachable `v === undefined` checks and no-op `.transform(Number)`

[kbpr21]

Summary

In packages/validation/api.ts, three threshold fields use a .refine() that
checks v === undefined, but this branch is unreachable, and a .transform(Number)
that is a no-op.

Affected fields:

• SearchRequestSchema.chunkThreshold
• SearchRequestSchema.documentThreshold
• Searchv4RequestSchema.threshold

Why the code is dead

Each field is defined as:

z.number().optional().default(0).refine((v) => v === undefined || (v >= 0 && v <= 1), {...}).transform(Number)

Zod applies .default() before downstream processing. When the field is omitted,
the parsed input is undefined, .default(0) replaces it with 0, and only then
does .refine() run. So v is always a number, never undefined, the
v === undefined || branch can never execute.

Additionally, .transform(Number) runs on a value that is already z.number(),
so it is a no-op.

Question before I open a PR

Happy to submit a PR removing the dead v === undefined checks and the redundant
.transform(Number) across all three fields. Before I do:

Is the .default(0) intentional? If "unset" is meant to be distinguishable from
an explicit 0, that may be worth addressing separately, otherwise I'll keep the
cleanup purely to the dead code and leave the defaults as-is.

[linear-code]

ENG-991