fix code unit/code point mix-up in cord substring length

[sahvx655-wq]

The cord overload of SubstringImpl in common/values/string_value.cc:745 passes size_code_points - start_code_units as the byte length to Subcord, mixing a code-point count with a byte offset. Use size_code_units so large strings backed by a cord return the correct bytes instead of a truncated or over-long range on multi-byte input.

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[jnthntatum]

Thanks for the contribution. Please sign the CLA and add a regression test and we can take a look.

[sahvx655-wq]

Thanks for the contribution. Please sign the CLA and add a regression test and we can take a look.

Done