fix: harden MiniReflect union handling for NONE and unknown union types

[Adyej999]

Problem

MiniReflect resolves union value offsets before fully confirming that the union discriminator represents a concrete union member.

For NONE and unknown/future union discriminator values, MiniReflect should avoid treating the union value as a concrete object. These cases can occur when inspecting unusual, malformed, or fuzzed FlatBuffer inputs through MiniReflect APIs such as:

FlatBufferToString()
IterateFlatBuffer()

The affected code path is in:

include/flatbuffers/minireflect.h

Impact

MiniReflect is used for FlatBuffer inspection and stringification. If union traversal is not defensive for non-concrete union states, MiniReflect can attempt invalid type-reference or value-offset handling while walking the buffer.

This makes MiniReflect less robust for edge-case union states and fuzzed inputs, especially for:

NONE union entries
unknown/future union discriminator values
size-prefixed buffer variants
custom IterationVisitor callbacks

Fix

This PR hardens MiniReflect union traversal so that union value offsets are not resolved before the union discriminator is known to represent a concrete union member.

The change:

- handles NONE union entries safely;
- handles unknown/future union types safely;
- avoids resolving union value offsets too early;
- avoids invalid MiniReflect type-reference lookup for non-concrete union entries;
- avoids forwarding resolved invalid union value pointers to IterationVisitor::Unknown().

Additional coverage

This PR also adds regression coverage for:

- single union NONE cases;
- vector-of-unions NONE cases;
- size-prefixed buffer variants;
- unknown/future union type handling.

A new MiniReflect fuzzer target is also added:

tests/fuzzer/flatbuffers_minireflect_fuzzer.cc

The fuzzer exercises MiniReflect APIs such as FlatBufferToString() and IterateFlatBuffer() over normal and size-prefixed inputs, including NONE and unknown/future union states.

Verification

Tested locally with:

flattests: ALL TESTS PASSED
minireflect_fuzzer: 1000 runs completed successfully
git diff --check: no whitespace errors