What does this PR do?:
Changes VMMethod::id() to return JMETHODID_NOT_WALKABLE instead of nullptr when the jmethodID cache is not yet populated for a klass. This prevents fillJavaFrame from taking the raw Method* pointer fallback path.

Motivation:
VMMethod::id() returned nullptr when a klass's jmethodID cache was not yet populated. fillJavaFrame treats nullptr as "store raw Method* pointer", deferring a dereference of that pointer to dump time (up to ~60s later per JFR chunk length). If HotSpot's redefinition cleanup reclaimed the underlying Method*/Symbol* metadata in that window, the unguarded memcpys in HotspotSupport::resolve() faulted on freed memory.

This fires on every RedefineClasses/RetransformClasses (routine, continuous — e.g. dd-trace-java agent retransformation): the cache is briefly empty between invalidation and repopulation, and a thread sampled in that window hits the nullptr path. Independent of the _force_jmethodID/fjmethodid config — that flag only controls the startup-time preload sweep, not this fallback.

Chain:

1. Agent retransforms a class → jmethodID cache empty
2. Concurrently-sampled frame → id() returns nullptr → fillFrameRaw stores raw Method* + isRawPointer flag
3. Dump time → resolveMethod sees isRawPointer(bci) → calls HotspotSupport::resolve() → unguarded memcpys fault on reclaimed metadata

Fix: Return JMETHODID_NOT_WALKABLE from all unpopulated-cache paths in VMMethod::id(). fillJavaFrame stores the sentinel without the raw flag; the dump thread maps it to the shared "unknown" method. No raw Method* dereference occurs. Covers 7 call sites (2 direct id() callers + 5 getMethodId()→validatedId()→id() callers).

Additional Notes:

• fillFrameRaw and isRawPointer left in place as defensive dead code
• HotspotSupport::resolve() still returns nullptr as the dump-time "unknown" signal — that contract with flightRecorder.cpp is unchanged
• OpenJ9/Zing paths untouched — they don't use fillJavaFrame/fillFrameRaw/isRawPointer

How to test the change?:

• HotspotMethodIdTest.RejectedMethodIdStaysNonRawAndResolvesToUnknown — passes (gtest)
• JMethodIDInvalidationStressTest — passes (integration, slow suite)
• All gtest suites pass; spotlessApply clean

For Datadog employees:

• This PR doesn't touch any of that.
• JIRA: PROF-15385