fix(ci): pin localstack to 4.14.0 (#6303)

* fix(ci): pin localstack to 4.14.0

LocalStack dropped their free community edition in March 2026
(version 2026.03.0), requiring an auth token to start. The
LOCALSTACK_ACKNOWLEDGE_ACCOUNT_REQUIREMENT workaround expired
April 6, 2026, breaking coverage CI permanently.

4.14.0 is the last free version (released Feb 26, 2026) and
works without authentication.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

* fix(ci): pin localstack version in .env.example

`latest` now requires auth since LocalStack dropped the free community
edition in March 2026. Pin to 4.14.0, the last free version.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>