Validate buttonless DFU advertising name length

[nrbrook]

Summary

• Validate buttonless DFU advertising name requests against the current write-with-response payload before writing.
• Preserve user-specified names instead of truncating them, and fail with DFUError.invalidAdvertisementName when the name is too long for the current connection.
• Document the conservative 18-byte limit for default ATT MTU and the runtime MTU-dependent behavior.

Problem

The existing implementation truncates a user-specified alternative advertising name to 20 bytes, matching the buttonless DFU service's maximum advertising name length. However, the full set-name write also includes the opcode and length byte, so a 20-byte name requires a 22-byte ATT write payload.

With the default ATT MTU, the write-with-response payload can be only 20 bytes. In that case, CoreBluetooth may send the oversized set-name command using ATT Prepare Write requests instead of a single ATT Write Request. Nordic's buttonless DFU set-name command is handled as a single write, and does not respond to prepare write, resulting in a connection timeout.

This change validates against the actual maximumWriteValueLength(for: .withResponse) at runtime. User-specified names that do not fit fail clearly instead of being truncated.

[CLAassistant]

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