Remove direct dependency on Azure SDK team hosted httpbin instance

[antkmsft]

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[antkmsft]

/azp run cpp - core

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[antkmsft]

/azp run cpp - core

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[azure-pipelines]

Azure Pipelines:
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[antkmsft]

/azp run cpp - core

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).

[antkmsft]

/azp run cpp - core

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines:
Successfully started running 2 pipeline(s).
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[Copilot]

Pull request overview

This PR removes the hard-coded dependency on the Azure SDK team-hosted httpbin endpoint in azure-core tests by switching to an environment-variable configured httpbin base URL, and conditionally skipping httpbin-dependent tests when it isn’t configured. It also updates developer documentation/spellchecking and adjusts CI coverage expectations accordingly.

Changes:

• Read the httpbin base URL from AZSDKCPPTEST_HTTPBIN_URL and expose IsEnabled() for tests to conditionally skip when not configured.
• Add GTEST_SKIP_() guards across multiple unit tests that previously assumed a fixed httpbin endpoint.
• Document the new test configuration and update CI/code coverage thresholds and spellchecker dictionary.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
sdk/core/ci.yml	Lowers line coverage target after making some tests conditional on httpbin configuration.
sdk/core/azure-core/test/ut/transport_policy_options.cpp	Makes httpbin-dependent tests skippable when the env var isn’t configured.
sdk/core/azure-core/test/ut/transport_adapter_base_test.hpp	Introduces env-var parsing/logging for httpbin URL and IsEnabled() helper.
sdk/core/azure-core/test/ut/transport_adapter_base_test.cpp	Adds skip guards to many transport adapter tests to avoid hard-coded httpbin dependency.
sdk/core/azure-core/test/ut/curl_options_test.cpp	Adds skip guards for curl-related tests that depend on httpbin.
sdk/core/azure-core/test/ut/curl_connection_pool_test.cpp	Adds skip guards for curl connection pool tests that depend on httpbin.
sdk/core/azure-core/test/ut/azure_libcurl_core_main_test.cpp	Switches the libcurl “globalCleanUp” test off httpbin.org hardcoding and adds skip guard.
sdk/core/azure-core/README.md	Documents AZSDKCPPTEST_HTTPBIN_URL requirement for running httpbin-based tests.
.vscode/cspell.json	Adds AZSDKCPPTEST to the dictionary.