fix(environment): prioritize --service-config flags over auto-detected stdin in get_edit_config

[AmariahAK]

Description

What was the issue?

railway environment edit --service-config <SERVICE> <PATH> <VALUE> silently discards the --service-config flags and exits 0 whenever stdin is not a TTY (scripts, CI, agent harnesses, or < /dev/null). No changes are committed, and in the default output mode nothing is printed — the command appears to succeed but does nothing.

Where was the issue?

src/commands/environment/edit.rs — the get_edit_config function checked !stdin_is_terminal before has_cli_flags. When stdin was non-TTY (even empty), it short-circuited into read_config_from_stdin, which returned an empty EnvironmentConfig::default(). The explicitly passed CLI flags were never reached.

Additionally, the empty-config early-return only printed "No changes to apply" when --json or an interactive TTY was active. Non-TTY, non-JSON callers got silence and exit code 0.

How was it fixed?

Two changes in get_edit_config:

1. Reordered the priority checks — moved has_cli_flags (explicit --service-config / --service-variable flags) to Priority 1, above the !stdin_is_terminal auto-detection block. Explicit user intent always wins over environment detection.
2. Added an eprintln! fallback — when the resulting config is empty and neither --json nor an interactive TTY is active, "No changes to apply" now prints to stderr so scripted callers can detect the no-op.

Why this approach?

• Matches user intent: someone passing --service-config meant to use it — the absence of a TTY shouldn't override that.
• Matches existing codebase patterns: the sibling function edit_services_select in new.rs already checks CLI flags first, then terminal state. get_edit_config was the outlier.
• Minimal diff: two blocks reordered, two lines added. No new functions, no new logic, no structural changes. Zero risk of breaking existing stdin JSON piping or interactive flows.

How to test locally

# Build
cargo build

# Reproduction (should have been broken — flags discarded, no change saved)
echo "" | ./target/debug/railway environment edit \
    --environment production \
    --service-config <SERVICE> deploy.preDeployCommand "echo fixed" \
    --stage --json

# Verify the change was staged
railway environment config --environment production --json

# Regression: piping JSON via stdin must still work
echo '{"services":{"<service-id>":{"deploy":{"preDeployCommand":"from-stdin"}}}}' | \
    ./target/debug/railway environment edit --environment production --stage --json

# Regression: interactive mode must still work
./target/debug/railway environment edit --environment production

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Fixes #1044

Original issue: #1044

[AmariahAK]

@codyde — could you add the release/patch label to this PR? I don't have permission as a fork contributor. Thanks!