feat: change default context window from 32000 to 128000 (#9673)

* feat: change default context window from 32000 to 128000

Update DEFAULT_CONTEXT_WINDOW and global_max_context_window_limit from
32000 to 128000. Also update all .af (agent files), cypress test
fixtures, and integration tests to use the new default.

Closes #9672

Co-authored-by: Sarah Wooders <sarahwooders@users.noreply.github.com>

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Co-Authored-By: Letta <noreply@letta.com>

* fix(core): update conversation manager tests for auto-created system message

create_conversation now auto-creates a system message at position 0
(from #9508), but the test assertions weren't updated. Adjust expected
message counts and ordering to account for the initial system message.

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Co-Authored-By: Letta <noreply@letta.com>

* fix(core): fix mock Anthropic models.list() to return async iterable, not coroutine

The real Anthropic SDK's models.list() returns an AsyncPage (with __aiter__)
directly, but the mock used `async def list()` which returns a coroutine.
The code does `async for model in client.models.list()` which needs an
async iterable, not a coroutine. Fix by making list() a regular method.

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---------

Co-authored-by: letta-code <248085862+letta-code@users.noreply.github.com>
Co-authored-by: Letta <noreply@letta.com>
Co-authored-by: Sarah Wooders <sarahwooders@gmail.com>

tests/test_agent_files/knowledge-base.af

@@ -44,7 +44,7 @@
     "provider_name": null,
     "provider_category": null,
     "model_wrapper": null,
-    "context_window": 32000,
+    "context_window": 128000,
     "put_inner_thoughts_in_kwargs": false,
     "handle": "anthropic/claude-3.5-sonnet",
     "temperature": 1.0,