fix(swift): scope JSON helpers to standalone output

[foobra]

Closes #3093

Swift standalone output can contain duplicate module-level newJSONDecoder and newJSONEncoder declarations when several generated files are compiled together. The original single-file fix used fileprivate, but the helpers were later moved to JSONSchemaSupport.swift for multi-file output and changed back to internal visibility.

This makes the visibility conditional:

• standalone output: fileprivate
• --multi-file-output: internal visibility in JSONSchemaSupport.swift

Added regression coverage for both rendering modes. Verified the unit tests, build, and Swift type-checking for generated standalone and multi-file output.

[github-actions]

Generated-output differences

314 files differ — 314 modified, 0 new, 0 deleted
1256 changed lines — +628 / −628

Open the generated-output report →

[github-actions]

Generated-output differences

314 files differ — 314 modified, 0 new, 0 deleted
1256 changed lines — +628 / −628

Open the generated-output report →

[foobra]

Relating this to #3105 (--nest-types), since both address multi-source standalone symbol collisions but at different scopes:

This PR (#3094) — scopes newJSONDecoder/newJSONEncoder to fileprivate in standalone output. Fixes the helper-symbol collision only. Child types stay at module scope, so repeated names like Image/Header/Payload across API models still collide. Keeps the flat type API (Image, not Root.Image).

#3105 (--nest-types) — opt-in option that nests child types under their top-level owner (Root.Image) and turns the helpers into static func members of the top-level type. Fixes both sibling-type and helper collisions, making each file fully self-contained. The trade-off is the type API changes (qualified Root.Image).

They're complementary rather than duplicates:

• Want to keep flat types but fix helper collisions → fix(swift): scope JSON helpers to standalone output #3094
• Want full multi-source isolation (types + helpers) and accept nested type API → Add Swift --nest-types option #3105

If only one is preferred, --nest-types (#3105) is the more complete fix for the collision class of issues (also covers #3104), but #3094 is the lower-impact change for users who can't adopt nesting.