fix(runtime): update shell session wire protocol

[priyasingh2111]

Issue #, if available:

Description of changes:

Updates the shell session wire protocol to simplify the connection handshake and remove deprecated features:

• Remove metadata frame handshake: Connection is ready immediately after WebSocket 101 upgrade — shellId is now read from the response header instead of waiting for a STATUS frame. Eliminates the blocking _read_metadata_frame() step and the _pending_frames queue.
• Remove encode_close() frame: close() no longer sends a CLOSE frame — the shell detaches and stays alive for the reconnect window.
• Simplify on_reconnect callback: No longer passes a reconnected: bool argument — callback takes no parameters.
• Remove reconnected and bytes_dropped attributes: No longer needed without the metadata frame flow.

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

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@5d4ca0d). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #640   +/-   ##
=======================================
  Coverage        ?   88.88%           
=======================================
  Files           ?      118           
  Lines           ?     9758           
  Branches        ?     1477           
=======================================
  Hits            ?     8673           
  Misses          ?      715           
  Partials        ?      370           

Flag	Coverage Δ
unittests	88.88% <100.00%> (?)

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

☔ View full report in Codecov by Harness.
📢 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.