fix: max spotter tx msg len for cellular

[towynlin]

What changed?

The Sofar Ocean server backend now supports the cellular-only queue for spotter_tx_data! This PR updates the checks for max payload length, since it varies depending on the telemetry channel. For Iridium SBD, the max payload length is 311 as before, but the cellular payload max is 1000 bytes.

Also, this PR fixes a memory leak. Previously, when payloads were > 311 bytes, we would allocate the buffer and return an error without freeing the buffer.

How does it make Bristlemouth better?

Enables Bristlemouth developers to manage telemetry costs by selectively using a cellular-only message queue on Spotter. These messages will be dropped and never sent if a cellular connection is not available.

Where should reviewers focus?

Please review all changes, less than 100 lines. I only modified one function and expanded its associated unit tests.

Checklist

• Add or update unit tests for changed code
• Ensure all submodules up to date. If this PR relies on changes in submodules, merge those PRs first, then point this PR at/after the merge commit
• Ensure code is formatted correctly with clang-format. If there are large formatting changes, they should happen in a separate whitespace-only commit on this PR after all approvals.

[towynlin]

Matt pointed out to me IRL that there was no memory leak before — I was mistaken on that point.