fix: s3session manager bug

[mehtarac]

Description

S3SessionManager with the default empty prefix (prefix="") produces S3 keys with a leading slash (e.g., /session_abc/agents/agent_default/agent.json). S3-compatible backends like MinIO normalize keys by stripping leading slashes on write, but the SDK reads using the original key with the leading slash. This mismatch causes read_agent() to return None, so the agent always takes the new-session branch on restore — silently discarding previous conversation history.

A related issue exists when users pass a prefix with a trailing slash (e.g., prefix="sessions/"), which produces double slashes in keys (sessions//session_abc/...).

# All of these now produce consistent, correct keys:
S3SessionManager(session_id="s1", bucket="b", prefix="")           # session_s1/...
S3SessionManager(session_id="s1", bucket="b", prefix="sessions")   # sessions/session_s1/...
S3SessionManager(session_id="s1", bucket="b", prefix="sessions/")  # sessions/session_s1/...
S3SessionManager(session_id="s1", bucket="b", prefix="/sessions/") # sessions/session_s1/...

Related Issues

#1863

Documentation PR

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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