* fix(proxy): build configured logging callbacks at boot so a broken callback fails startup instead of the first request
Startup already built every configured callbacks, success_callback and
failure_callback entry, but through the per-request helper that swallows
every construction error. A callback whose SDK could not be imported (for
example langfuse outside the supported version range) stayed a string, so
the first request on each worker rebuilt it inside function_setup and
returned a 500.
Startup now builds them with a variant that re-raises ImportError naming
the callback, which fails boot. Other construction errors keep the old
non-blocking behavior, and callbacks added at runtime keep the lenient path.
* refactor(proxy): build the configured callback replacements in one pass
Collect the startup-built callbacks with a generator and a frozen mapping
instead of a mutable dict filled in a loop, and drop a docstring that
restated the function body
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Co-authored-by: yuneng-jiang <yuneng@berri.ai>
