[typespec-ts] Serialization/deserialization refinements (additional properties, dates, error/body split)

[JialinHuang803]

Overview

Umbrella tracking issue for serialization/deserialization refinements in the TypeScript emitter (@azure-tools/typespec-ts, modular). Migrated from Azure/autorest.typescript, which is being retired in favor of this repo.

Work items

1. Additional properties ser/deser follow-ups

Improve ser/deser logic for both legacy and non-legacy formats:

• Date in additional properties — for ...Record<utcDateTime>, the generated serializer calls serializeRecord(item.additionalProperties ?? {}) without a value serializer, so Date values are emitted raw instead of being encoded to the wire format. (Verified locally.)
• clientName / wireName inconsistency in additional-properties keys/values
• multipart/form-data with additional properties (note: overlaps the multipart umbrella [typespec-ts] Complete multipart/form-data and multipart content-type support #4757 — de-dup there)
• Optional unixTimestamp value of 0 — for an optional @encode("unixTimestamp") date, the deserializer generates !item["value"] ? item["value"] : new Date(item["value"] * 1000); when the value is 0, !0 is truthy so the raw 0 is returned instead of new Date(0). (Required fields are handled correctly.)
• Migrated from Additional properties follow-ups in Modular autorest.typescript#3122

2. General ser/deser refinement

• Fix record-of-date serialization issues
• Clean up the unit tests under packages/typespec-ts/test/modularUnit/serializeUtil
• Migrated from serialization/deserialization refine autorest.typescript#2997

3. Separate error and body deserialization (nice-to-have)

• _xxxDeserialize currently bundles the error-status check and body deserialization in a single function. Extract the error-checking part into a reusable helper that can be called without consuming result.body, so callers (e.g. Storage submitBatch) don't do unnecessary work or consume the body.
• Migrated from Deserialization: separate error and body deserializations autorest.typescript#3797

Notes

• Test coverage is tracked as part of each fix above, not as standalone issues.