fix: pass lifespan to starlette

[tonyxwz]

*Issue #, lifespan is super useful in managing server resources

Description of changes:

just allow BedrockAgentCoreApp to take lifespan and pass the value to starlette.

Agent application might need to start MCP servers which are processes. Since agentcore is a single tenant environment (isolated by session ids), the MCP servers can be launch once in the lifespan, and reused.

@asyncontextmanager
async def mcp_server_manager():
  async with stdio_client("duckduckgo-search") as (read, write):
    async with ClientSession(read, write) as session:
      await session.initialize()
      yield


app = BedrockAgentCoreApp(lifespace=mcp_server_manager)

if __name__ == "__main__":
  app.run()

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@255f950). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #51   +/-   ##
=======================================
  Coverage        ?   88.14%           
=======================================
  Files           ?       16           
  Lines           ?     1544           
  Branches        ?      231           
=======================================
  Hits            ?     1361           
  Misses          ?       89           
  Partials        ?       94           

Flag	Coverage Δ
unittests	88.14% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[siwachabhi]

Thanks for raising the PR, could you please add unit tests for the changes

[siwachabhi]

Covering this PR here: #67 added some unit tests