refactor(framework): Disable gRPC Control API by default

[jafermarq]

No description provided.

[Copilot]

🟢 Approval recommended

The behavior change is localized, covered by a targeted test, and does not introduce risky control-flow or interface changes.

Pull request overview

This PR refactors SuperLink startup so the gRPC Control API is disabled by default and can be explicitly enabled via an environment variable, aligning the default runtime posture with a more restricted exposed surface.

Changes:

• Gate SuperLinkLifespan._start_control_api() behind FLWR_INTERNAL_GRPC_CONTROL_API=1.
• Add a regression test verifying the gRPC Control API only starts when explicitly opted in.
• Update CLI comments/log messages to reflect the new behavior.

Review Notes

Critical issues

• None found.

Simplicity/readability suggestions

• The new “Control HTTP API” start log is redundant because only a single FastAPI/Uvicorn server is started (same host/port); consider consolidating logs.
• One updated comment still implies FastAPI serves only “Runtime HTTP”, but the FastAPI app also includes Control routes.

Consistency concerns

• The "1" env-var opt-in pattern matches other toggles already used in the codebase (e.g., update check/event log toggles).

Should this PR be split?

• No.

Verdict

• Good change overall; a couple of small messaging cleanups suggested in review comments.

File summaries

File	Description
framework/py/flwr/superlink/cli/flower_superlink.py	Gate gRPC Control API startup behind an opt-in env var; adjust CLI comments/logging.
framework/py/flwr/superlink/cli/flower_superlink_test.py	Add test coverage for the new opt-in behavior.
framework/py/flwr/common/constant.py	Introduce FLWR_INTERNAL_GRPC_CONTROL_API constant for the new toggle.

Review details

Suppressed comments (1)

framework/py/flwr/superlink/cli/flower_superlink.py:530

• Two INFO logs announce starting separate Runtime and Control HTTP APIs on the same host/port, but only one Uvicorn/FastAPI server is started here. This is redundant and can confuse operators into thinking there are multiple listeners; consider logging a single combined message.

    log(
        INFO,
        "Starting the SuperLink Runtime HTTP API on %s:%s.",
        lifespan_config.host,
        lifespan_config.port,
    )
    log(
        INFO,
        "Starting the SuperLink Control HTTP API on %s:%s.",
        lifespan_config.host,
        lifespan_config.port,
    )

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

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