Feature: Allow AGENT_ENDPOINT env var to customize URL on Agent card

[esnible]

Summary

This continues PR #144 adding support for Agent Card URL customization to other examples that incorrect offered http://0.0.0.0:8000.

A follow-up PR in the Kagenti repo will make Kagenti supply the $AGENT_ENDPOINT value.

Related issue(s)

rossoctl/rossoctl#776

[mrsabath]

Review Summary

The PR applies the AGENT_ENDPOINT env var override pattern consistently across 12 agent examples, continuing work from PR #144. The trailing-slash normalization (.rstrip("/") + "/") is a nice touch.

Issue: 4 files missing import os

These files use os.getenv() but do not have import os, causing NameError at runtime (confirmed by CI lint failure F821):

• a2a/file_organizer/src/file_organizer/agent.py
• a2a/reservation_service/src/reservation_service/agent.py
• a2a/slack_researcher/a2a_agent.py
• a2a/weather_service/src/weather_service/agent.py

The other 8 files are fine -- 6 have the import added in this PR, and 2 already had it from PR #144.

Fix: Add import os to each of the 4 affected files.

Areas reviewed: Python, Security, Commits
Commits: 1 commit, signed-off: yes
CI status: lint failing (F821 Undefined name os)

[esnible]

@mrsabath Your review is correct about a2a/slack_researcher/a2a_agent.py, although lint had independently found the problem and I was fixing it as you conducted your review. The other three files DO have the import os. Perhaps re-review or help me to understand the problem.

[mrsabath]

Follow-up Review

Previous REQUEST_CHANGES issues resolved:

• slack_researcher/a2a_agent.py -- fixed in commit 8d15b8d (added import os)
• file_organizer, reservation_service, weather_service -- already had import os on the fork branch (my earlier check against upstream main was misleading)

All CI checks pass (lint, test, security scans, DCO). LGTM.