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

## Overview

Umbrella tracking issue for serialization/deserialization refinements in the TypeScript emitter (`@azure-tools/typespec-ts`, modular). Migrated from [Azure/autorest.typescript](https://github.com/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`, 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 Azure/typespec-azure#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 Azure/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 Azure/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 Azure/autorest.typescript#3797

## Notes

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