Migrate FastAPI on_event handlers to lifespan

[FawwadAhmed11]

FastAPI deprecated on_event in 0.109; the upper pin <0.116 was added defensively in case it was removed. It hasn't been removed through 0.128+, but the pin still blocks users who need a newer FastAPI. This PR migrates off on_event and drops the pin.

Closes #1960

What does this PR do?

Replaces the three on_event handlers in http_api.py and parallel_http_api.py with a single asynccontextmanager-based lifespan function. Adds an async startup_hook() extension point on HttpInterface that ParallelHttpInterface overrides for its Redis-dispatch setup. Drops the fastapi<0.116 upper bound from requirements/_requirements.txt.

All existing behavior is preserved: shutdown telemetry flush, preload model thread launch (still gated on PRELOAD_MODELS/PINNED_MODELS), and parallel Redis checker thread setup all work identically — just triggered by lifespan instead of on_event.

Related Issue(s): Fixes #1960

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Testing

• I have tested this change locally
• I have added/updated tests for this change

Test details:

• python -m py_compile passes on both modified Python files (syntax and closure validity confirmed)
• make check_code_quality passes clean — black, isort, flake8 all green
• CI will run the full unit test suite

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have updated the documentation accordingly (if applicable)

Additional Context

The lifespan function is defined inside HttpInterface.__init__ so it can close over local variables (description, should_preload, model_init_state, initialize_models) that are needed at startup/shutdown time. Python resolves these at call time, not definition time, so there is no NameError risk even though some variables are defined later in __init__.

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	pypi/​fastapi@​0.115.14 ⏵ 0.136.3	+1

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