fix: throw clear error for non-existent listSearchableFields

[vjymisal0]

What?

Adds validation that throws a clear InvalidConfiguration error when admin.listSearchableFields on a collection config references a field name that does not exist on that collection.

Why?

Fixes #14904.

Previously, referencing a non-existent field name in admin.listSearchableFields produced no error at all in pnpm dev, and in production builds it "imploded" the admin List View search with no usable error message - the actual failure ("The following path cannot be queried: <field>") only ever showed up in server logs, never reaching the user.

How?

Added validateListSearchableFields in packages/payload/src/collections/config/listSearchableFields.ts, mirroring the existing validateUseAsTitle precedent in the same directory. It flattens the collection's top-level fields and checks each entry in admin.listSearchableFields (skipping the implicit id field, and only checking the top-level segment of dotted/relationship paths) exists on the collection, throwing InvalidConfiguration naming the offending field and collection slug if not.

This is called from sanitizeCollection right after validateUseAsTitle, so it runs unconditionally at config-sanitization time in both dev and production - no more environment-dependent failure mode.

Test plan

Added packages/payload/src/collections/config/listSearchableFields.spec.ts (mirrors useAsTitle.spec.ts), covering:

• throws on a non-existent field
• does not throw when all fields exist
• does not throw on the default id field
• does not throw when listSearchableFields is undefined

Note: pnpm install in this sandbox environment was intermittently failing on Windows temp-path EPERM/network timeout issues while I prepared this PR, so I could not get a green local vitest run to paste here. The test code itself directly follows the existing, currently-passing useAsTitle.spec.ts pattern in the same file, using the same sanitizeCollection harness.

Fixes #14904

[vjymisal0]

Update: dependencies finished installing in my sandbox, so I was able to run the tests locally after all.

npx vitest run packages/payload/src/collections/config/listSearchableFields.spec.ts packages/payload/src/collections/config/useAsTitle.spec.ts --project unit

 Test Files  2 passed (2)
      Tests  12 passed (12)

All 4 new tests pass, and the existing useAsTitle.spec.ts suite (whose pattern this PR's test mirrors) is unaffected.