Fix EventStream initial message handling to prevent hangs

[rcoh]

Fixes #4435

Previously, codegen would always call try_recv_initial() for event streams, which would block waiting for an initial-request or initial-response message. This caused hangs when operations didn't have modeled initial messages.

Changes:

Runtime (aws-smithy-http & aws-smithy-legacy-http):

• recv() now filters out initial-request/initial-response messages and stores them in a separate buffer
• try_recv_initial() checks the buffer first before blocking
• Initial messages are discarded if never explicitly read
• Added try_recv_initial_with_preprocessor() to legacy version for SigV4 support

Codegen:

• Server: Only calls try_recv_initial() when httpBindingResolver.handlesEventStreamInitialRequest() returns true
• Client: Only calls try_recv_initial_response() when there's a parser for non-event-stream output members

Tests:

• Added test_no_hang_without_initial_message() to verify operations without modeled initial messages don't hang
• Added test_waits_for_initial_message_when_modeled() to verify operations with modeled initial messages still block correctly
• Added wait_for_response_ready() to ManualEventStreamClient for testing connection readiness

Documentation:

• Updated AGENTS.md with EventStream initial message handling behavior
• Added conditional mutability pattern to AGENTS.md

Motivation and Context

Description

Testing

Checklist

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the applies_to key.
• For changes to the AWS SDK, generated SDK code, or SDK runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "aws-sdk-rust" in the applies_to key.

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