refactor(profiling): add explicit casts to avoid narrowing conversions

[KowalskiThomas]

Description

https://datadoghq.atlassian.net/browse/PROF-13775

Use explicit static_cast where implicit narrowing conversions occur:

• danger.cc: Cast page calculations to safe_memcpy_return_t
• mirrors.cc: Cast table_size calculation to ssize_t
• greenlets.cc: Cast count to int before adding
• stack_renderer.cpp: Cast microsecond_t to int64_t for push_* calls
• vm.cc: Cast size_t to off_t for ftruncate, riovcnt to int for pwritev
• uploader.cpp: Cast len to std::streamsize for write()
• frame.cc: Cast offsetof result to int
• sampler.cpp: Cast uint64_t to double before subtraction

Fixes clang-tidy bugprone-narrowing-conversions warnings.

[cit-pr-commenter-54b7da]

Codeowners resolved as

ddtrace/internal/datadog/profiling/stack/echion/echion/timing.h         @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack/include/stack_renderer.hpp     @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack/src/echion/greenlets.cc        @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack/src/echion/mirrors.cc          @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack/src/echion/threads.cc          @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack/src/stack_renderer.cpp         @DataDog/profiling-python