fix(apps): open app URL without config

[tomr-a5]

What

Allow fly apps open -a <app> to open the default https://<app>.fly.dev/ URL when flyctl cannot load app config.

Why

Supplying -a currently still fails if no local/remote app config can be loaded, which makes the app flag feel ignored for a command whose main job is to open the app URL.

How

• Keep using config-derived URLs when local or remote app config is available
• Ignore local config that belongs to a different app than the selected app
• Fall back to the default Fly app hostname only when app config is unavailable
• Print a warning before falling back, while keeping the raw remote-load error in debug logs
• Share relative-URI handling across both paths
• Add focused tests for fallback, warning output, relative URI handling, local config, mismatched local config, remote config, missing app name, and no-public-service behavior

Tests

go test ./internal/command/apps -run TestRunOpen -count=1 -v
go test ./internal/command/apps -count=1
go test ./internal/command/... -count=1
go test ./... -run '^$' -count=1
go test ./... -count=1

Fixes #4215

[Copilot]

Pull request overview

Enables fly apps open -a <app> to still open the app in a browser when no local/remote app config can be loaded, by falling back to the default https://<app>.fly.dev/ URL (with a warning), while preserving config-derived URL behavior when config is available.

Changes:

• Add fallback behavior and warning output when remote app config can’t be loaded, and ignore mismatched local config when -a selects a different app.
• Refactor URL-opening logic into a shared helper to consistently apply relative-URI handling.
• Add focused unit tests covering fallback, warning output, relative URI parsing, local/remote config paths, and no-public-service behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/command/apps/open.go	Adds config-load fallback to default app hostname, ignores mismatched local config, and centralizes URL opening/relative-URI handling.
internal/command/apps/open_test.go	Adds targeted tests validating fallback behavior, warning output, relative URI handling, and local vs remote config selection.

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