Add polling options, service endpoint, CBOR error handling, and SDK updates

[JeromySt]

Performance: Fast retry policy, tunable polling, structured error handling, and SDK updates

Motivation

The Azure Code Transparency service returns 503 TransactionNotCached when a newly registered entry hasn't propagated to the read replicas yet — typically resolved in well under 1 second. However, the server's Retry-After: 1 header causes the SDK's default retry logic to wait a full second between attempts, adding unnecessary latency to the hot path of entry registration and receipt retrieval. This PR eliminates that bottleneck and adds several supporting features.

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🚀 MstTransactionNotCachedPolicy — Fast 503 retry pipeline policy (new)

A custom HttpPipelinePolicy that intercepts 503 TransactionNotCached responses and performs aggressive, in-pipeline retries before the SDK's standard retry logic kicks in.

Parameter	Default	Effect
retryDelay	250ms	Fixed interval between retry attempts
maxRetries	8	Maximum retry attempts (total window ≈ 2 seconds)

How it works:

1. Intercepts responses matching all of: HTTP 503 + GET method + URI path contains /entries/ + CBOR body with TransactionNotCached error code
2. Re-sends the request at 250ms intervals until success or max retries exhausted
3. Non-matching requests (POST, different paths, different error codes) pass through untouched to the SDK's normal retry infrastructure

Key design decisions:

• Positioned at HttpPipelinePosition.PerRetry — runs inside the SDK's retry loop, minimizing total latency
• Parses response body as RFC 9290 CBOR problem details to verify the error code (case-insensitive)
• Buffers non-seekable response streams to ensure the body remains readable across retries
• Parse failures (invalid CBOR, stream I/O errors) gracefully fall through to SDK retry — no false positives

Configuration via extension method:

options.ConfigureTransactionNotCachedRetry();                         // 250ms, 8 retries (default)
options.ConfigureTransactionNotCachedRetry(TimeSpan.FromMilliseconds(100)); // faster
options.ConfigureTransactionNotCachedRetry(maxRetries: 16);           // longer window

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⏱️ MstPollingOptions — Tunable receipt polling (new)

Controls how MakeTransparentAsync polls for the long-running operation receipt.

Property	Type	Behavior
PollingInterval	TimeSpan?	Fixed interval between polls (e.g., 250ms)
DelayStrategy	DelayStrategy?	Custom back-off (exponential, jitter, etc.) — takes precedence

• Both null → SDK default exponential back-off
• PollingInterval set → WaitForCompletionAsync(interval, ct)
• DelayStrategy set → Operation<T>.DelayStrategy assignment (overrides PollingInterval)
• RegisterCommand and VerifyCommand now default to 250ms fixed polling

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📋 CborProblemDetails — RFC 9290 Concise Problem Details parser (new)

Parses CBOR-encoded error responses per RFC 9290:

CBOR Key	Field	Description
-1	Type	URI reference identifying the problem type
-2	Title	Human-readable summary
-3	Status	HTTP status code
-4	Detail	Human-readable explanation
-5	Instance	URI reference for this specific occurrence
other	Extensions	Any additional keys captured as a dictionary

Also accepts string keys ("type", "title", etc.) for interoperability. Handles HalfPrecisionFloat CBOR values consistent with CwtClaims.cs.

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🔥 MstServiceException — Structured error wrapping (new)

Wraps Azure.RequestFailedException with parsed CBOR problem details:

catch (RequestFailedException rfEx)
{
    throw MstServiceException.FromRequestFailedException(rfEx);
    // rfEx.ProblemDetails.Title → "TransactionNotCached"
    // rfEx.ProblemDetails.Detail → "Transaction ... is not yet cached"
    // rfEx.StatusCode → 503
}

• Factory method inspects Content-Type for cbor, extracts and parses the response body
• Falls back to generic wrapping if CBOR parsing fails
• ToString() includes all parsed fields and extensions for diagnostics

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🔌 TransparencyService.ServiceEndpoint — Base class URI property (new)

• Auto-derived from CodeTransparencyClient via reflection (best-effort)
• Explicit URI override available via constructor parameter
• Available to all TransparencyService implementations

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📦 Azure SDK package updates

Package	Before	After
Azure.Core	1.50.0	1.51.1
Azure.Identity	1.17.0	1.18.0
Azure.Security.CodeTransparency	1.0.0-beta.5	1.0.0-beta.8

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🧪 Azure.Core.TestCommon — Test infrastructure (new)

Vendored from azure-sdk-for-net test infrastructure (Azure.Core 1.51.1 era):

• MockResponse, MockTransport, MockRequest — controlled HTTP responses for testing Azure SDK clients
• AsyncGate, AsyncValidatingStream, DictionaryHeaders — supporting utilities
• All classes marked [ExcludeFromCodeCoverage]

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🗑️ Removed

• CtsCommandBase (332 lines) and CtsCommandBaseLoggingTests (482 lines) — superseded by the new polling and retry infrastructure

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Test coverage

Area	Tests	Coverage
MstTransactionNotCachedPolicy	23	Constructor validation, pass-through for non-matching requests, retry-until-success, retry exhaustion, case-insensitive matching, sync/async paths
MstPollingOptions	22	Property defaults, constructor wiring, all MstTransparencyService overloads
CborProblemDetails	41	Null/empty/invalid input, integer keys, string keys, extensions, partial fields, HalfPrecisionFloat
MstServiceException	covered in CborProblemDetails tests	FromRequestFailedException, BuildErrorMessage, ToString, StatusCode
MstTransparencyService	+1 new	MakeTransparentAsync wraps RequestFailedException in MstServiceException
Total new	87+

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Summary for reviewers

The core performance improvement is the MstTransactionNotCachedPolicy — by retrying at 250ms instead of the server's 1-second Retry-After, entry receipt retrieval that previously took 2–4 seconds now typically completes in under 1 second. Combined with 250ms LRO polling intervals (replacing SDK default exponential back-off), end-to-end registration latency is significantly reduced.

All new code has structured CBOR error handling, cancellation token support, and comprehensive test coverage.