[Fix][OCaml] Properly handle free-forms and byte arrays

[sir4ur0n]

Fixes #21307
Fixes #21312
Fixes a few more problems I uncovered in the process:

• Fix support for free-forms (e.g., additionalProperties: true)
• Fix support for byte arrays
• Support decimal type
• Test the whole fake-petstore test suite for OCaml

💡 I tried to split into more atomic commits. These are not perfect, but should help review formatting (e.g., mustache indentation) changes vs actual fixes

💡 This MR/branch is built on top of #21679, so I suggest reviewing/merging #21679 first.

On top of the 2 test suites in this PR, I also manually tested this branch against the OpenAPI spec for which we generate OCaml code at my company, and it works.

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@cgensoul

[sir4ur0n]

Closing this MR as #21798 which contained all the commits was merged.