[Epic] - Fix issues found when generating OpenAI

[joheredi]

Blocking: Issues blocking code gen

• [Codegen] [JS] Missing isUnexpected Overload #2013
• [Codegen] Throw rest errors in the right shape #1972
• [Codegen] RangeError: Maximum call stack size exceeded at getResponseMapping #1982
• [Codegen] Serialization Incomplete For Polymorphic Input #2139
• [Customization] [customization] Imports not getting resolved correctly azure-sdk-for-js#26947
• [Customization] [dev-tool] Customization does not merge export statements azure-sdk-for-js#26799
• [Customization] [dev-tool] customization duplicates constant enums azure-sdk-for-js#26798
• [Customization] [customization] Does Not Create Nested Folders azure-sdk-for-js#27614

Issues:

• [CodeGen] Missing Imports For Models Under Rest Folder #2118
• [Codegen] Support multipart/form-data for file uploads #2018
• [Codegen] [JS] Send Request Body Missing Deserialization for Object #2024
• [Codegen] isUnexpected should handle the streaming responses #2008
• [Codegen] The version in the user-agent string is overwritten by beta.1 #1973
• [Customization] [Customization] Duplicate Copyright Header azure-sdk-for-js#27837
• [Customization] [Customization] Should not Export from Generated Directory azure-sdk-for-js#27838
• [Customization] [Customization] Duplicate Exports From The Same File azure-sdk-for-js#27839
• [Customization] [Customization] Missing Docstring After Customization azure-sdk-for-js#27858

Improvements

• [Codegen] Better modularization of request/response logic #1971
• [Codegen] [Modular] Unnecessary undefined insertions for optional properties #1990
• [Codegen] [JS] Extensible Enum Generation #2011
• [Codegen] [CodeGen] Add a way to hide operations #2119
• [Customization] [customization] Need some way to remove select generated bits from the customized code azure-sdk-for-js#26949