Skip union members when discriminator is unmatched

[Muon]

Changelog

Fix corrupted read when a union discriminator does not match any case in the union's definition.

Docs

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Description

MessageReader silently produces corrupted data if it encounters an unknown union discriminator.

This is because it leaves the read cursor in place just after the discriminator, causing the caller to believe it finished reading the union. The caller would then start reading its next data from the middle of the union member.

In this PR, we instead skip the rest of the union if possible, or throw an error if not.

Tests have been added to cover this with mutable and final types. I've checked that they all fail without these changes.

Fixes: FG-14472

[linear]

FG-14472 OMG IDL union parsing fails on missing case discriminator

[james-rms]

Can you add to the description how the new test relates to the failing test data provided to us by a customer? do we know the test encodes the message in the same fashion that the user did?

[Muon]

I didn't derive these tests directly from the customer data. They were written to target this bug specifically.

However, I've confirmed by running the app locally that the customer's data fails to deserialize without this patch, and succeeds with it.

Although it succeeded, I do not know whether the data was deserialized correctly. Checking that by manual inspection of the message bytes would take a long time because the message is huge and complicated. (Not to mention it would be error-prone...). We can investigate further later, but I think we should ship this now because it's definitely fixing a bug and it seems to work for the customer's data.