feat: add compressMetadata / decompressMetadata to compression.ts

[ZacLou]

Overview

Implements #619 — adds compressMetadata and decompressMetadata to src/compression.ts with JSON + base64url round-trip and size validation.

Changes

src/compression.ts

• compressMetadata(obj: Record<string, unknown>, maxBytes?: number): string — serializes to JSON then encodes as base64url (no padding =). Throws SdkError with CONTRACT_REJECTED if the resulting base64 string exceeds maxBytes (default 512).
• decompressMetadata(encoded: string): Record<string, unknown> — decodes base64url and parses JSON. Throws SdkError with CONTRACT_REJECTED on invalid base64 input, invalid JSON, or non-object JSON.

src/index.ts

• Exports compressMetadata and decompressMetadata.

test/metadataCompression.test.ts

• Round-trip correctness (simple, nested, empty object)
• Oversized payload throws CONTRACT_REJECTED
• Custom maxBytes argument respected
• Invalid base64 input throws CONTRACT_REJECTED
• Valid base64 but invalid JSON throws CONTRACT_REJECTED
• Decoded JSON that is not an object (array, string, number) throws CONTRACT_REJECTED

Acceptance Criteria Checklist

• compressMetadata(obj: Record<string, unknown>): string exported — serializes to JSON then encodes as base64url (no padding =)
• decompressMetadata(encoded: string): Record<string, unknown> exported — decodes base64url and parses JSON; throws SdkError with CONTRACT_REJECTED code on invalid input
• compressMetadata throws SdkError with CONTRACT_REJECTED code if the resulting base64 string exceeds maxBytes (default 512 bytes, configurable via second argument)
• Round-trip: decompressMetadata(compressMetadata(obj)) deep-equals obj for any JSON-serialisable input
• Both exported from src/index.ts
• Unit tests: round-trip correctness, oversized payload throws, invalid base64 input throws, empty object round-trips

Closes #619