Deprecate keep_warm in favor of AWS-native cold start solutions

[monkut]

Summary

• Change keep_warm default from true to false
• Add deprecation warning during deploy/update/schedule when keep_warm is enabled
• Add DeprecationWarning in keep_warm_callback handler
• Update README documentation to mark keep_warm as deprecated

Context

Investigation in #1445 confirmed that keep_warm:

• Does not reduce cold start duration — init times are identical (~1,050ms) with or without it
• Only reduces cold start frequency by keeping one execution environment alive
• Only keeps 1 environment warm — concurrent requests still hit full cold starts

AWS-native alternatives are superior:

• SnapStart: 45-72% faster cold starts
• Provisioned Concurrency: eliminates cold starts entirely

The feature remains functional for users who explicitly opt in. Full removal planned for a future major version.

Closes #1451

Test plan

• New tests verify deprecation warning appears when keep_warm: true
• New tests verify no warning when keep_warm: false
• New tests verify keep_warm_callback emits DeprecationWarning
• New tests verify default is now false
• Pre-commit hooks pass (isort, black, flake8, mypy)
• Existing test suite unaffected

[coveralls]

coverage: 74.692% (+0.04%) from 74.651% — deprecate-keep-warm into master

[dennybiasiolli]

Small change: what about removing the keep_warm key/value in example/zappa_settings.json?

diff --git a/example/zappa_settings.json b/example/zappa_settings.json
index 74c8f051..28b31f93 100644
--- a/example/zappa_settings.json
+++ b/example/zappa_settings.json
@@ -1,7 +1,6 @@
 {
     "dev_event": {
        "project_name": "dev_event_zappa_test_flask_app",
-       "keep_warm": false,
        "debug": true,
        "log_level": "DEBUG",
        "events": [{
@@ -20,7 +19,6 @@
     },
     "dev_api": {
        "project_name": "dev_api_zappa_test_flask_app",
-       "keep_warm": false,
        "debug": true,
        "log_level": "DEBUG",
        "aws_region": "us-west-2",

[github-actions]

Hi there! Unfortunately, this PR has not seen any activity for at least 90 days. If the PR is still relevant to the latest version of Zappa, please comment within the next 10 days if you wish to keep it open. Otherwise, it will be automatically closed.