Share AIRFLOW_VAR_* env-override helper between HTTP and coordinator Go SDK clients

[jason810496]

Nice to have modularization, not-blocking for 3.3.0 release.

---

Dependency

• Wait until Go-SDK: Add concurrent-safe coordinator comms, log handler, and client #67317 merges — the coordinator-mode duplication lands in that PR, so the dedup work can only start once both copies exist on main.

Background

The Go SDK has two independent implementations of the same Python-parity rule that an environment variable named AIRFLOW_VAR_<UPPER(key)> short-circuits GetVariable and is returned in preference to the supervisor-stored value (matching airflow.models.variable.Variable.get).

The HTTP-backed client uses an unexported helper:

// go-sdk/sdk/client.go
func variableFromEnv(key string) (string, bool) {
    return os.LookupEnv(VariableEnvPrefix + strings.ToUpper(key))
}

func (*client) GetVariable(ctx context.Context, key string) (string, error) {
    // TODO: Let the lookup priority be configurable like it is in Python SDK
    if env, ok := variableFromEnv(key); ok {
        return env, nil
    }
    ...
}

The coordinator-mode client inlines the same logic and carries a TODO calling out the duplication:

// go-sdk/pkg/execution/client.go
func (c *CoordinatorClient) GetVariable(ctx context.Context, key string) (string, error) {
    // TODO: this duplicates variableFromEnv in sdk/client.go. The env-first
    // precedence is part of the SDK contract, so both clients should share a
    // single helper (e.g. an exported sdk.VariableFromEnv) instead of two
    // independent copies that can drift.
    if env, ok := os.LookupEnv(sdk.VariableEnvPrefix + strings.ToUpper(key)); ok {
        return env, nil
    }
    ...
}

The env-first lookup order is part of the public Go SDK contract; two independently maintained copies can silently drift, particularly if the lookup precedence is made configurable later (as the HTTP-side TODO notes).

What needs to happen

1. Promote variableFromEnv to an exported helper on the sdk package — e.g. sdk.VariableFromEnv(key string) (string, bool) — so the env-override logic has a single canonical implementation.
2. Update go-sdk/sdk/client.go to call the exported helper.
3. Update go-sdk/pkg/execution/client.go CoordinatorClient.GetVariable to call the same exported helper and drop the inlined os.LookupEnv block.
4. Remove the duplication TODO in pkg/execution/client.go.

Acceptance criteria

• Only one implementation of the AIRFLOW_VAR_* env-override lookup exists in the Go SDK source tree.
• Both sdk.client and execution.CoordinatorClient call the shared helper.
• Existing tests (TestCoordinatorClientGetVariableEnvOverride and any HTTP-side equivalent) continue to pass without modification, or are updated to exercise the shared helper directly if duplication of the test coverage no longer makes sense.

Context

• Originating review: self-review thread on Go-SDK: Add concurrent-safe coordinator comms, log handler, and client #67317 (coordinator comms + client PR).
• HTTP-side helper: go-sdk/sdk/client.go (variableFromEnv).
• Coordinator-side duplicate (lands with Go-SDK: Add concurrent-safe coordinator comms, log handler, and client #67317): go-sdk/pkg/execution/client.go CoordinatorClient.GetVariable.
• Related project board: AIP-72 (addendum): Go-SDK (view)