Refactor endpoint response object handling

[bluwy]

Changes

• Make callEndpoint return a response directly, even converting simple object form to a response so we don't duplicate work.
• For SSG build, we still need to return the object form. callEndpoint now accepts an extra returnObjectFormIfAvailable param to return that. Mostly for compat as we might remove object form support in Astro 3.0

Testing

Existing tests should pass.

Docs

n/a. internal refactor.

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[bluwy]

Note: it's easier to review with whitespaces off. Toggle the comments below for extra context.

Also marking this draft as it's breaking something.

[bluwy]

Well this didn't really go well. Looks like in dev and prod, we handle simple objects with encoding differently.

Dev: Use Node's Buffer.from
Prod: Use TextEncoder (only works with utf8 strings)

Dev behaviour encompasses prod behaviour, which is fine because it covers static mode that writes the content to a file with different encodings. I don't think this is a nice refactor until if we decide to remove the simple object syntax. Will comment on that linear issue instead.