feat(go): add OAuth token override support

[tjb9dc]

Description

Refs #11029

Implements OAuth token override support for the Go SDK generator, mirroring the feature from the TypeScript generator PR #11029. When enabled via the oauthTokenOverride config option, users can provide a pre-generated bearer token directly via the WithToken option function, bypassing the OAuth client credentials flow.

Link to Devin run: https://app.devin.ai/sessions/f7ea32bafbe64688bced47359d9fb67c
Requested by: [email protected] (@tjb9dc)

Changes Made

• Added oauthTokenOverride to Go v2 custom config schema (BaseGoCustomConfigSchema.ts)
• Added OAuthTokenOverride field to Go v1 custom config and generator Config structs
• Implemented OAuth token override in Go v1 generator (sdk.go):
  • Adds Token string field to RequestOptions struct when OAuth auth scheme is present and config is enabled
  • Adds WithToken option function for setting the token
  • Adds logic in ToHeader() to set Authorization: Bearer <token> header
• Updated all Go generator entry points (fern-go-sdk, fern-go-model, fern-go-fiber) to pass the new parameter
• Updated versions.yml with changelog entry for v1.19.0

Updates Since Last Revision

• Fixed CI build failure by adding OAuthTokenOverride parameter to NewConfig calls in fern-go-model/main.go and fern-go-fiber/main.go (the parameter is passed through but not used by these generators)

Testing

• TypeScript lint checks passed (pnpm run check)
• Go compilation verification (Go not installed in environment - relying on CI)
• Seed tests for Go SDK

Human Review Checklist

• Verify Go code compiles correctly (CI should confirm this)
• Verify generated code produces correct WithToken option when oauthTokenOverride: true is configured
• Consider adding/updating seed tests for OAuth token override feature
• Note: Go v2 generator has OAuth marked as TODO - only config option was added, no implementation

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