[Repo Assist] Add OmitNullFields static parameter to JsonProvider (closes #1245)

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Summary

Adds an OmitNullFields static parameter to JsonProvider so users can opt out of serializing optional fields as null — instead omitting them entirely from the generated JSON.

Addresses #1245. Requested by @dsyme in this comment.

Usage

// Default (backward-compatible): optional fields are serialized as null
type ColorProvider = JsonProvider<"""[{"color": "Red", "code": 15}, {"color": "Green"}]""", SampleIsList = true>
let v1 = ColorProvider.Root(color = "Blue", code = None)
// v1.ToString() → {"color": "Blue", "code": null}

// With OmitNullFields=true: None fields are omitted entirely
type ColorProviderOmit = JsonProvider<"""[{"color": "Red", "code": 15}, {"color": "Green"}]""", SampleIsList = true, OmitNullFields = true>
let v2 = ColorProviderOmit.Root(color = "Blue", code = None)
// v2.ToString() → {"color": "Blue"}

Root Cause

JsonRuntime.CreateRecord always serialized every field in the constructor args array, converting None options to JsonValue.Null. There was no way to tell it to skip null-valued fields.

Fix

• JsonRuntime.fs: Added CreateRecordOmitNulls(properties, cultureStr) — same as CreateRecord but filters out entries where the value converts to JsonValue.Null.
• JsonGenerator.fs: Added OmitNullFields: bool to JsonGenerationContext. The record constructor code generation now calls CreateRecordOmitNulls when OmitNullFields = true.
• JsonProvider.fs: Added OmitNullFields: bool static parameter (default false) at index 13.
• TypeProviderInstantiation.fs: Updated test helper to include the new parameter.
• JsonProvider.fs (tests): Added 3 tests verifying OmitNullFields behavior.

Design Notes

• Default is false — no change in behavior for existing users; fully backward-compatible.
• OmitNullFields=true applies to all record constructors in the generated type hierarchy.
• Non-optional fields are never null, so they are unaffected by this parameter.

Test Status

• ✅ FSharp.Data.Tests — 262/262 pass (259 existing + 3 new OmitNullFields tests)
• ✅ FSharp.Data.DesignTime.Tests — 486/487 pass (1 skipped: network-blocked test, pre-existing)
• ✅ fantomas formatting verified — no changes needed

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