SDK variables: optional env-var fallback (IR envVar) + support in Go/C#/Ruby/PHP generators (Twilio subaccount routing) #18048

Summary

Twilio (fern-demo/twilio-core-fern-config) needs the official-SDK "act on behalf of a subaccount" behaviour: parent-account credentials + a single client-level account SID that is substituted into every v2010 {AccountSid} path, defaulting to the parent account when not set (twilio(sid, token, { accountSid }), Client(sid, token, account_sid=), Twilio.init(sid, token, accountSid)).

Fern already has SDK variables (x-fern-sdk-variables / x-fern-sdk-variable, Fern definition variables:), but two gaps blocked using them for this:

1. No fallback – a variable was always a required constructor argument. Parent-account callers would have to pass targetAccountSid explicitly; the official SDKs default it to the auth account SID (TWILIO_ACCOUNT_SID).
2. Only TypeScript, Python and Java implemented IR variables – Go, C#, Ruby (v2) and PHP silently ignored x-fern-sdk-variable and kept the path parameter on every method.

Design

• IR VariableDeclaration.envVar?: string (IR 67.29.0).
• Fern definition: variables: { targetAccountSid: { type: string, env: TWILIO_ACCOUNT_SID } }; OpenAPI: x-fern-sdk-variables: { targetAccountSid: { type: string, x-fern-env: TWILIO_ACCOUNT_SID } }; bind with x-fern-sdk-variable: targetAccountSid on path params.
• Generators: a string variable with envVar becomes an optional client option; explicit value > environment variable; if a bound endpoint is called with neither set, the SDK raises a clear "<variable> is required" error. Variables without envVar stay required. Bound path parameters are removed from method signatures / request types / reference docs; one value per client (matches official Twilio semantics).

PRs

• IR/CLI: feat(cli): add env var fallback for SDK variables (VariableDeclaration.envVar) #18022 (envVar on VariableDeclaration, Fern definition env, OpenAPI x-fern-env)
• C#: feat(csharp): SDK variables with env var fallback in the C# SDK generator #18034 (new SDK-variable support + env fallback)
• PHP: feat(php): SDK variables with env var fallback in the PHP SDK generator #18038 (new SDK-variable support + env fallback)
• TypeScript: feat(typescript): optional SDK variables with env var fallback #18029 (optional option + env fallback)
• Java: feat(java): SDK variables with env var fallback default to System.getenv and are validated at build() #18030 (builder method + System.getenv fallback, validated in generated setVariables)
• Go v1 + v2: feat(go): SDK variables with env var fallback in Go v1 and Go v2 generators #18033 (option.With<Variable>, os.Getenv fallback)
• Ruby v2: feat(ruby): SDK variables with env var fallback in the Ruby v2 SDK generator #18035 (new SDK-variable support + env fallback)
• Python: blocked until fern_fern_ir_v67==67.29.0 is published to the private PyPI index (the IR publish job for 67.29.0 failed and needs a maintainer rerun; @fern-fern/ir-sdk@67.29.0 on npm is fine). Implementation is ready to open once the package is there.

Follow-up (Twilio side)

• fern-demo/twilio-core-fern-config#157 adds targetAccountSid to the core v2010 override (191 {AccountSid} path params bound; account-management {Sid} paths stay explicit).
• After the generator releases above, bump the pinned generator versions in that repo and regenerate/verify all 7 SDKs (TS targetAccountSid, Python target_account_sid, Java .targetAccountSid(...), Go option.WithTargetAccountSid(...), C# ClientOptions.TargetAccountSid, Ruby target_account_sid:, PHP targetAccountSid:).

Not in scope: per-call override of a bound variable alongside the client default (the official SDKs' client.api.v2010.accounts(sid)... escape hatch) — Fern drops the parameter from the method once it is bound.