[mlir][bytecode] Use getChecked<T>() in bytecode reading to avoid crashes (#186145)

When the bytecode type callback (test-kind=2) calls iface->readType()
for every builtin type, complex types like MemRefType could crash
because the generated reading code used get<T>() which asserts on
invalid parameters, rather than getChecked<T>() which returns null
gracefully.

This change:
- Adds a getChecked<T>() free function helper in
BytecodeImplementation.h that calls T::getChecked(emitError, params)
(no-context form) when a specific override exists, otherwise falls back
to get<T>(). The with-context second branch is intentionally omitted to
avoid instantiating StorageUserBase::getChecked<Args> for types that
only inherit the base template (e.g. ArrayAttr), which would require
complete storage types unavailable in the bytecode reading TU.
- Updates BytecodeBase.td default cBuilder for
DialectAttribute/DialectType to use getChecked<> instead of get<>.
- Updates all custom cBuilder strings in BuiltinDialectBytecode.td.
- Updates the no-args codegen case in BytecodeDialectGen.cpp.
- Adds a regression test in bytecode_callback_with_custom_type.mlir.

Fixes #128308

Assisted-by: Claude Code