Implement updated retry behavior gated behind an internal flag

[ubaskota]

⚠️ This change is internal-only. External users are not affected and should not depend on the gated behavior.

Description of changes:
Implements the updated retry behavior for standard retry mode. Changes include updated exponential backoff with service specific scale factors, revised retry quota costs (14 for non-throttling, 5 for throttling), DynamoDB-specific max attempts (4) and base backoff (25ms), long-polling operation backoff on quota exhaustion, and x-amz-retry-after header support. All changes are gated behind a STANDARD_RETRY_MODE_VERSION flag imported from an internal customization, ensuring no behavior change for external users. The default retry mode is also updated to standard for internal users via the same flag.

Tests:

• Added tests to verify the updated retry behavior behind the gated flag works as expected. All new tests pass.
• Verified that existing tests for the current retry behavior continue to pass.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 90.52632% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.56%. Comparing base (ee748e4) to head (11c05c2).
⚠️ Report is 164 commits behind head on develop.

Files with missing lines	Patch %	Lines
botocore/retries/standard.py	89.02%	9 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3678      +/-   ##
===========================================
- Coverage    92.59%   92.56%   -0.04%     
===========================================
  Files           68       68              
  Lines        15634    15767     +133     
===========================================
+ Hits         14477    14595     +118     
- Misses        1157     1172      +15     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[SamRemis]

Suggested change

	class TestNewRetriesEnvironmentVariable(unittest.TestCase):
	@mock.patch.dict(os.environ, {'AWS_NEW_RETRIES_2026': 'true'})
	def test_env_var_true_enables_new_retries(self):
	importlib.reload(configprovider)
	self.assertTrue(configprovider.NEW_RETRIES_ENABLED)
	@mock.patch.dict(os.environ, {'AWS_NEW_RETRIES_2026': 'false'})
	def test_env_var_false_disables_new_retries(self):
	importlib.reload(configprovider)
	self.assertFalse(configprovider.NEW_RETRIES_ENABLED)
	@mock.patch.dict(os.environ, {}, clear=False)
	def test_no_env_var_uses_default(self):
	os.environ.pop('AWS_NEW_RETRIES_2026', None)
	importlib.reload(configprovider)
	self.assertFalse(configprovider.NEW_RETRIES_ENABLED)
	class TestNewRetriesEnvironmentVariable(unittest.TestCase):
	@mock.patch.dict(os.environ, {'AWS_NEW_RETRIES_2026': 'true'})
	def test_env_var_true_enables_new_retries(self):
	self.assertTrue(configprovider._resolve_new_retries())
	@mock.patch.dict(os.environ, {'AWS_NEW_RETRIES_2026': 'false'})
	def test_env_var_false_disables_new_retries(self):
	self.assertFalse(configprovider._resolve_new_retries())
	def test_no_env_var_uses_default(self):
	with mock.patch.dict(os.environ, {}, clear=False):
	os.environ.pop('AWS_NEW_RETRIES_2026', None)
	self.assertFalse(configprovider._resolve_new_retries())

Fact check me here, but I think this will fix the failing CI along with my other suggestion since this doesn't rely on the global module state.

[SamRemis]

My bad, it still wasn't working. Found some similar tests that use BaseEnvVar, and confirmed this worked on my fork.

[SamRemis]

Suggested change

	class TestNewRetriesEnvironmentVariable(unittest.TestCase):
	@mock.patch.dict(os.environ, {'AWS_NEW_RETRIES_2026': 'true'})
	def test_env_var_true_enables_new_retries(self):
	importlib.reload(configprovider)
	self.assertTrue(configprovider.NEW_RETRIES_ENABLED)
	@mock.patch.dict(os.environ, {'AWS_NEW_RETRIES_2026': 'false'})
	def test_env_var_false_disables_new_retries(self):
	importlib.reload(configprovider)
	self.assertFalse(configprovider.NEW_RETRIES_ENABLED)
	@mock.patch.dict(os.environ, {}, clear=False)
	def test_no_env_var_uses_default(self):
	os.environ.pop('AWS_NEW_RETRIES_2026', None)
	importlib.reload(configprovider)
	self.assertFalse(configprovider.NEW_RETRIES_ENABLED)
	class TestNewRetriesEnvironmentVariable(BaseEnvVar):
	def test_env_var_true_enables_new_retries(self):
	self.environ['AWS_NEW_RETRIES_2026'] = 'true'
	self.assertTrue(configprovider._resolve_new_retries())
	def test_env_var_false_disables_new_retries(self):
	self.environ['AWS_NEW_RETRIES_2026'] = 'false'
	self.assertFalse(configprovider._resolve_new_retries())
	def test_no_env_var_uses_default(self):
	self.assertFalse(configprovider._resolve_new_retries())

[SamRemis]

Suggested change

	import importlib
	import os