Enable CA1062 (validate arguments of public methods) to catch missing null-argument guards at build time

[daviburg]

Summary

Enable CA1062 ("Validate arguments of externally visible methods") as a build-enforced analyzer rule so missing null-argument guards on public/protected APIs are caught deterministically at build time instead of ad hoc during code review.

Motivation

Missing ArgumentNullException guards on public entry points are a recurring manual review nit (including from the Copilot PR reviewer). Each occurrence currently costs a review round-trip: reviewer spots it, author adds ArgumentNullException.ThrowIfNull(...), re-review. CA1062 catches the entire class at compile time, consistently, before a PR is ever opened.

This rule is disabled by default in the SDK's current analysis mode (AnalysisMode=Default), so it never fires today even though EnableNETAnalyzers=true and TreatWarningsAsErrors=true are set in eng/build/Engineering.props.

Evidence (from a local AnalysisMode=All build)

CA1062 fires on hand-written production code that skips argument validation, e.g. in ConnectorClientBase:

• CallConnectorAsync<TResponse>(HttpMethod method, ...) — method not validated before use.
• CallConnectorAsync(HttpMethod method, ...) (no-body overload) — same.

New code such as ConnectorTriggerPayload already validates via ArgumentNullException.ThrowIfNull, so it is clean — which shows the rule aligns with the direction the codebase is already moving.

Scope / considerations

• Generated code is in scope. The generated clients are produced by our own CodefulSdkGenerator (BPM repo) and are a core part of the value proposition; they must not ship missing guards. Enabling CA1062 will surface any generated entry points lacking validation — if it flags generated code, the fix belongs in the generator's emit templates, not a per-file suppression.
• Severity: start as warning (which TreatWarningsAsErrors escalates to an error) or suggestion for a soft rollout, then promote.
• Rollout: enable via .editorconfig (dotnet_diagnostic.CA1062.severity = warning) rather than raising the whole AnalysisMode, to keep the change targeted and avoid the ~17k unrelated diagnostics that AnalysisMode=All produces.
• Note: CA1062 has known limitations (it may not track validation performed in a called helper), so expect a few false positives to suppress with justification.

Acceptance criteria

• dotnet_diagnostic.CA1062.severity set in .editorconfig.
• Existing hand-written violations fixed (add ArgumentNullException.ThrowIfNull).
• Generated-code violations (if any) fixed at the generator, or a documented, justified suppression policy agreed.
• Build stays green with the rule enforced.

Filed from analysis on PR #196; not part of that PR's scope.