chore (ts-client): validate required Electric headers on response

[kevin-dp]

Fixes https://github.com/electric-sql/electric/issues/1950

Questions:

• Are the headers only required on 2XX responses or also on other responses? They are currently only being checked on 2XX responses.
• Are we checking all required headers? Currently we check electric-offset, electric-handle, and electric-schema

[thruflo]

Can we also validate an electric-cursor header when we receive a 204 in response to a live request?

Not having the cursor param can result in re-requesting to the same URL.

[thruflo]

Plus can we please write a section in the https://electric-sql.com/docs/guides/troubleshooting guide about this and include a link to it in the error message?

Basically we would want to explain that if headers are missing then your proxy needs to make sure it includes them, maybe with a code sample showing how (Kyle has some proxy code that clones the headers).

I can write this if you'd like.

[KyleAMathews]

Checking just 200s for headers seems fine as it covers all the headers.

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[kevin-dp]

@KyleAMathews Can you have another review at this PR to see if you approve the changes? There are some required headers that are checked on all 2xx responses. There are also some headers that are required only in live mode and some only in non-live mode. I also check those on 2xx responses.

I will open a separate docs PR before merging this one.

[kevin-dp]

For reference, this is the accompanying docs PR: https://github.com/electric-sql/electric/pull/1976