Fixed issues validating tracing pages which were converted to page iterators

[LarryOsterman]

• Re-add the span when converting from Init state on a continuation token;
• Cleaned up lifetime of MockSpan to have it be dropped when the actual span goes out of scope (also verified that the same "end on drop" behavior occurs with an OpenTelemetry Span).
• Added a Debug requirement for the C parameter so it can be printed successfully.
• Added a simple test for verifying the spans coming from an ItemIterator converted immediately to a PageIterator.

Created as Draft so it doesn't conflict with releases of various non-core crates.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

azure_core

[Copilot]

Pull Request Overview

This PR fixes issues with tracing/instrumentation when using page iterators in the Azure SDK, particularly when converting from an ItemIterator to a PageIterator or when resuming pagination from a continuation token.

Key changes:

• Re-added span creation when converting from Init state with a continuation token in the pager logic
• Refactored MockSpan to properly manage span lifecycle with automatic cleanup on drop
• Added test coverage for page iterator telemetry scenarios including immediate conversion and rehydration

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
sdk/core/azure_core/src/http/pager.rs	Fixed span creation logic to establish public API span when entering Init state (both initial and rehydrated); added custom Debug impl for State<C> using AsRef<str>
sdk/core/azure_core_test/src/tracing.rs	Refactored MockSpan into wrapper/inner pattern for proper drop semantics; added documentation for ExpectedInstrumentation fields
sdk/keyvault/azure_security_keyvault_secrets/tests/secret_client.rs	Added new test for page iterator telemetry; updated existing tests to use env! macros for package info; removed #[ignore] attribute from rehydration test
sdk/core/azure_core/src/http/mod.rs	Added Sanitizer to public exports
sdk/keyvault/assets.json	Updated test recording asset tag