Add HTTP protocol types, exception hierarchy, and MockHTTPResponse

[mwdd146980]

What does this PR do?

Add three new utility modules to datadog_checks_base:

• http_exceptions: Transport-agnostic HTTP exception hierarchy (HTTPError, HTTPStatusError, HTTPTimeoutError, etc.)
• http_protocol: Protocol classes defining the HTTP client interface (HTTPClientProtocol, HTTPResponseProtocol)
• http_testing: MockHTTPResponse for use in integration tests, replacing the requests-specific MockResponse from datadog_checks_dev

These modules are purely additive and have no dependencies on existing code. They enable the ongoing httpx migration by providing library- agnostic abstractions that work with both requests and httpx backends.

Motivation

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[datadog-prod-us1-4]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 82.82%
• Overall Coverage: 87.02%

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[codecov]

Codecov Report

❌ Patch coverage is 82.81938% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.54%. Comparing base (94024ce) to head (6686cd3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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[dd-octo-sts]

Validation Report

Validation	Status
agent-reqs	✅
ci	✅
codeowners	✅
config	✅
dep	✅
http	✅
imports	✅
integration-style	✅
jmx-metrics	✅
labeler	✅
legacy-signature	✅
license-headers	✅
licenses	✅
metadata	✅
models	✅
openmetrics	✅
package	✅
readmes	✅
saved-views	✅
version	✅

View full run

[mwdd146980]

• Migrate conftest.py MockResponse fixture chains to MockHTTPResponse #23226
• Batch migrate integration tests from MockResponse to MockHTTPResponse #23207
• Widen production except clauses for httpx migration #23046
• Migrate mock_http fixture and test files to MockHTTPResponse #23300
• Add HTTP protocol types, exception hierarchy, and MockHTTPResponse #23299 👈 (View in Graphite)
• master

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