feat(typescript): add customSubpathExports configuration option

[jsklan]

Description

Refs: Request from judah@buildwithfern.com (@jsklan)

Adds a new customSubpathExports configuration option for the TypeScript SDK generator. This allows SDK maintainers to define custom subpath exports in the generated package.json, enabling modular packages where optional features can be imported via subpaths (e.g., import { BedrockClient } from 'cohere-ai/aws').

Link to Devin run: https://app.devin.ai/sessions/63cecf1472274d33b8facb2cc909c130

Changes Made

• Added Zod schema for customSubpathExports in TypescriptCustomConfigSchema.ts supporting both simple string exports and complex conditional exports with types/import/require/default
• Added type definition to SdkCustomConfig.ts and parsing in SdkGeneratorCli.ts
• Passed config through SdkGenerator.Config to TypescriptProject hierarchy
• Modified SimpleTypescriptProject.generatePackageJson() to merge custom exports after the main "." export and before "./package.json"
• Added seed test fixture exhaustive/custom-subpath-exports demonstrating the feature
• Updated versions.yml with changelog entry for version 3.46.0

Testing

• Seed test exhaustive:custom-subpath-exports passes
• TypeScript compilation passes
• Verified generated package.json contains merged exports in correct order

Human Review Checklist

• Verify the Zod schema handles all expected input formats (simple strings, nested objects)
• Confirm BundledTypescriptProject doesn't need this feature (currently only SimpleTypescriptProject supports it)
• Check the generated package.json exports order is correct: . → custom exports → ./package.json

[devin-ai-integration]

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