Add a public StreamApiException constructor

[harlan]

Summary

StreamApiException is a public, catch-and-branch type — integrators are expected to catch it, inspect StatusCode/Code, and branch via the public StreamApiExceptionExtensions.Is* helpers (IsNoAccessToChannelsError, IsRateLimitExceededError, etc.). But its only constructor is internal and takes the internal APIErrorInternalDTO, so integrators can't construct one to unit-test their own error handling.

This means there's no supported way to write a test like "when a channel-hydration call throws a 403 / code 70, my code drops the channel from the local list instead of rethrowing" — you can't produce the exception to feed your handler without reaching into SDK internals (e.g. an InternalsVisibleTo patch against a vendored copy).

This PR adds a public constructor over the type's already-public fields. APIErrorInternalDTO stays internal.

Change

public StreamApiException(
    int statusCode,
    int code,
    string errorMessage = null,
    string moreInfo = null,
    string duration = null,
    IReadOnlyDictionary<string, string> exceptionFields = null)

Both this and the existing DTO constructor funnel through a single private constructor and a shared BuildMessage helper, so the Exception.Message format is identical regardless of how the exception is built (keeps log/crash-report grouping consistent). Every parameter maps 1:1 to an existing public property; nothing internal is exposed.

Why this shape

• A plain public constructor (vs. a ForTesting-style factory) is idiomatic for an Exception subclass and reads naturally at call sites. Hand-constructing a representative API error is legitimately useful beyond tests (fakes, mocks, replay tooling).
• This matches the signature suggested in the support thread; APIErrorInternalDTO is kept internal as requested.

Integrator impact (before → after)

Before — only possible by granting InternalsVisibleTo to the SDK and using the internal DTO:

// Requires internal access to both StreamApiException's ctor and APIErrorInternalDTO
private static StreamApiException NoAccessToChannelsException()
    => new(new APIErrorInternalDTO
    {
        StatusCode = StreamApiException.NoAccessToChannelsErrorHttpStatusCode, // 403
        Code       = StreamApiException.NoAccessToChannelsErrorStreamCode,     // 70
        Message    = "channels match your query but cannot be returned",
    });

After — public API only:

private static StreamApiException NoAccessToChannelsException()
    => new(
        StreamApiException.NoAccessToChannelsErrorHttpStatusCode, // 403
        StreamApiException.NoAccessToChannelsErrorStreamCode,     // 70
        "channels match your query but cannot be returned");

Testing

• Verified the refactor produces byte-identical Exception.Message output to the previous inline code across all error shapes (null message/moreInfo, null/empty/multi-entry exceptionFields).
• Existing CI runtime test suite covers the rest.

Changelog

Added an entry under Unreleased / Features.

[sierpinskid]

All tests passed:

[sierpinskid]

Thank you for you contribution!