chore: rename internal modules with _ prefix per Python convention

[planetf1]

Problem

Two modules are implementation details that don't follow Python's _ prefix convention for private modules. The _ prefix is how Python signals "this is not public API" — to users, IDEs, type checkers, and static analysis tools. This is a code hygiene issue; the fact that it was surfaced via the doc generator is incidental.

mellea/formatters/granite/intrinsics/json_util.py — used only internally within the same package. intrinsics/__init__.py does not export it. Should be _json_util.py.

mellea/telemetry/backend_instrumentation.py — telemetry/__init__.py has an explicit __all__ that does not include it, yet six backend modules import it directly via cross-package relative imports, bypassing the package interface. Should be _backend_instrumentation.py.

There is a secondary question for backend_instrumentation: should its functions (instrument_generate_from_raw, start_generate_span etc.) be exported through telemetry/__init__.py so backends access them via the package interface rather than reaching into a submodule the package doesn't expose? That would be the cleaner design but is a separate decision.

Neither name appears in any __init__.py __all__, so there is no public API breakage. All callers are internal import sites within mellea/ that need updating.

Side effect

The doc generator has a _CONFIRMED_INTERNAL_MODULES hardcoded workaround specifically because these two modules aren't _-prefixed. Once renamed, that workaround can be removed.