Phase-3A OSAL capability contract cleanup

[grape-wzy]

Description

Refine the Phase-3A OSAL capability contract so the public API and private backend contract stay limited to capability reporting.

Related Issue(s)

• grape-wzy/monar_desing_guidance#10

Type of Change

• Refactor
• Documentation

Checklist

• I have read and followed the design guidance in grape-wzy/monar_desing_guidance.
• My code follows the coding rules and directory structure conventions.
• All new and existing tests pass.
• I have added or updated documentation if necessary.
• I have verified that no RTOS/HAL/vendor headers leak into public headers.

Validation

• make test passes on host
• cmake --build --preset host-debug passes
• ctest --preset host-debug passes
• cmake --build --preset stm32f407-debug passes (if applicable)

Summary of Changes

• keep the public OSAL capability query API focused on mn_osal_get_capabilities() and mn_osal_has_capability()
• remove the extra time/sleep OSAL entry points and matching private backend hooks from the previous broader Phase-3 work
• add MN_OSAL_CAP_ISR_CONTEXT so backends can describe ISR-context detection support explicitly
• document that capability queries are deterministic before and after mn_osal_init()
• update host OSAL tests to cover pre-init and post-init capability behavior

Notes

This PR is intentionally small and does not start Phase-3B runtime/service APIs or any BSP/device expansion work.