Python: Quick Redis sample fix

[giles17]

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request refactors the Redis chat message store session sample to make the Redis URL configurable via environment variable instead of being hardcoded. This improves the sample's flexibility for users who need to connect to remote or authenticated Redis instances.

Changes:

• Introduced a REDIS_URL constant that reads from the environment variable with a fallback to the local default
• Replaced all hardcoded "redis://localhost:6379" occurrences with the REDIS_URL constant
• Updated the prerequisites message to reflect the new configuration option