fix(csharp): emit millisecond precision for nullable datetime params in mock tests

[patrickthornton]

Closes FER-10694

Summary

Generated C# mock-server tests for nullable date-time query/header parameters were producing WithParam("since", "2024-01-15T09:30:00Z") (no millis) while the SDK runtime serializes DateTime with yyyy-MM-ddTHH:mm:ss.fffK and actually sends 2024-01-15T09:30:00.000Z. WireMock requires exact match → 404.

Root cause: getDateTime in MockEndpointGenerator.ts only unwrapped container.optional. When the IR wraps a date-time in container.nullable (common with respect-nullable-schemas: true + coerce-optional-schemas-to-nullable: false), it returned undefined and the test fell back to the raw value.jsonExample string, which omits millis.

Example

Found while debugging auth0-teams-csharp-sdk CI: Activity.ListTest failed with Auth0.TeamsApiException : Error with status code 404. The IR for the since parameter walked as container.optional → container.nullable → named.alias → primitive.datetime. The nullable step short-circuited the recursion.

Before/after for that generated test:

// Before
.WithParam("since", "2024-01-15T09:30:00Z")
.WithParam("until", "2024-01-15T09:30:00Z")

// After
.WithParam("since", "2024-01-15T09:30:00.000Z")
.WithParam("until", "2024-01-15T09:30:00.000Z")

Fix:

case "container": {
    const container = exampleTypeReference.shape.container;
    if (container.type === "optional") {
        return container.optional == null ? undefined : this.getDateTime(container.optional);
    }
    if (container.type === "nullable") {
        return container.nullable == null ? undefined : this.getDateTime(container.nullable);
    }
    return undefined;
}

Test plan

• pnpm turbo run compile --filter @fern-api/fern-csharp-sdk — clean
• pnpm turbo run test --filter @fern-api/fern-csharp-sdk — 41/41 pass
• Re-ran seed on auth0-teams-csharp config; dotnet test --filter FullyQualifiedName~Activity.ListTest now passes 2/2 (was 0/2)
• No existing csharp-sdk seed snapshots use the bare Z form, so no fixture churn expected

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SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-05-20T05:20:23Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
csharp-sdk	square	67s (n=5)	105s (n=5)	79s	+12s (+17.9%)

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).
Baseline from nightly runs on main (latest: 2026-05-20T05:20:23Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-05-20 21:13 UTC