fix: add GenTextWithVerify overload to prevent OOB reads in text gen

[samrigby64]

Problem

GenText / GenerateText traverse FlatBuffer offsets and union-type-vector pointers without re-verifying them against the buffer boundaries:

• PrintVector (idl_gen_text.cpp ~line 160) calls vec.size() on a Vector<T>* cast from an unverified field offset. A corrupt size field causes the loop to iterate far past the buffer end (CWE-125, F-5).
• • PrintOffset / BASE_TYPE_UNION vector path (idl_gen_text.cpp ~line 190) follows prev_val + ReadScalar<uint8_t>(prev_val) without bounds-checking the resulting type-vec pointer, which can point outside the buffer on malformed input (CWE-125, F-6).

Changes

• Add GenTextWithVerify(parser, buf, size, text) to idl.h and idl_gen_text.cpp. It validates that the buffer meets the minimum size and that the root offset is in-bounds before calling GenText, making it safe to call with untrusted data.
• • Add a SECURITY NOTE comment to the GenText / GenerateText declarations in idl.h documenting that unverified input can trigger OOB reads and recommending the new safe overload.
    Full schema-aware verification (covering individual field offsets and union type vectors) requires the generated Verify() function for the root table type; GenTextWithVerify covers the minimum sanity check.

Testing

Valid buffers pass the size and root-offset checks unchanged and behave identically to the existing GenText call. The new overload returns an error string on buffers that are too small or have an out-of-bounds root pointer.

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