Replace usages of RxJava2 in testing with Reactor Test

[alzimmermsft]

Description

Replace usage of RxJava2 in Cosmos tests with equivalent logic using Reactor Test given the SDK uses Reactor and not RxJava.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR migrates Cosmos SDK tests from RxJava2 to Reactor Test, aligning the testing framework with the SDK's actual reactive implementation (Reactor). The migration replaces io.reactivex.subscribers.TestSubscriber with reactor.test.StepVerifier and reactor.test.subscriber.TestSubscriber, removes RxJava2 dependencies, and updates related comments.

Key Changes:

• Replaced RxJava2 TestSubscriber with Reactor StepVerifier for most test validations
• Migrated to Reactor TestSubscriber for backpressure testing scenarios
• Updated comments referencing RxJava to reference Reactor
• Removed RxJava2 dependency from test scope in pom.xml files
• Applied code quality improvements (static final modifiers, long literal style, redundant cast removal)

Reviewed changes

Copilot reviewed 38 out of 38 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
StoreReader.java	Updated comments from RxJava references to Reactor, removed redundant casts, improved string joining logic
ConsistencyWriter.java	Updated comments from RxJava to Reactor references
DatabaseForTest.java	Made logger and TIME_FORMATTER static final, reordered imports
TopQueryTests.java	Migrated TestSubscriber to StepVerifier for query continuation token testing
TestSuiteBase.java	Replaced RxJava2 TestSubscriber validation patterns with StepVerifier patterns
SinglePartitionDocumentQueryTest.java	Migrated continuation token test to StepVerifier with AtomicReference pattern
ReadFeedExceptionHandlingTest.java	Simplified error verification using StepVerifier.expectNextCount().verifyError()
QueryValidationTests.java	Migrated query result collection to StepVerifier with collectList()
ParallelDocumentQueryTest.java	Replaced TestSubscriber with StepVerifier for continuation token queries
OrderbyDocumentQueryTest.java	Migrated multiple query tests to StepVerifier, changed from long literals with 'l' to 'L'
OffsetLimitQueryTests.java	Migrated continuation token tests to StepVerifier
FeedRangeQueryTests.java	Migrated query result collection to StepVerifier
BackPressureTest.java	Migrated to Reactor TestSubscriber for backpressure testing with proper API usage
BackPressureCrossPartitionTest.java	Migrated backpressure tests to Reactor TestSubscriber
FetcherTest.java	Simplified validation using StepVerifier.consumeNextWith()
DocumentProducerTest.java	Migrated producer tests to StepVerifier, identified missing assertion
StoreReaderTest.java	Replaced RxJava2 validation helpers with StepVerifier-based validation
StoreReaderDotNetTest.java	Migrated validation methods to StepVerifier
RntbdTransportClientTest.java	Simplified failure validation using StepVerifier
ReplicatedResourceClientTest.java	Migrated with proper Exceptions.unwrap() for error validation
ReplicatedResourceClientRetryWithTest.java	Replaced TestSubscriber validation with StepVerifier
ReplicatedResourceClientPartitionSplitTest.java	Migrated validation methods, missing Exceptions.unwrap()
QuorumReaderTest.java	Changed long literals from lowercase 'l' to uppercase 'L', migrated validations
MetadataRequestRetryPolicyTests.java	Migrated ShouldRetryResult validation to StepVerifier
HttpTransportClientTest.java	Simplified validation methods using StepVerifier
GatewayAddressCacheTest.java	Migrated address cache validation to StepVerifier
ConsistencyWriterTest.java	Migrated timeout tests to StepVerifier, added Reactor TestSubscriber import
ConsistencyReaderTest.java	Migrated validation methods to StepVerifier, reordered imports
WebExceptionRetryPolicyTest.java	Migrated retry policy validation to StepVerifier
TestSuiteBase (impl).java	Migrated base test validation methods to StepVerifier
RxGatewayStoreModelTest.java	Simplified failure validation using StepVerifier
RetryUtilsTest.java	Migrated with proper Exceptions.unwrap() for error handling
ClientRetryPolicyTest.java	Migrated retry result validation to StepVerifier
RetryContextOnDiagnosticTest.java	Migrated response validation methods to StepVerifier
CosmosMultiHashTest.java	Migrated continuation token tests to StepVerifier
azure-cosmos-tests/pom.xml	Removed RxJava2 dependency
azure-cosmos-encryption/pom.xml	Removed RxJava2 dependency
external_dependencies.txt	Removed RxJava2 from external dependencies