fix(credentials): read runpodctl top-level apikey as fallback

[justinwlin]

flash could not authenticate from a ~/.runpod/config.toml written by runpodctl. runpodctl writes a top-level apikey key with no [default] profile table, but get_api_key() only consulted runpod-python's profile-based get_credentials(), which returns no key in that case — so flash reported "No RunPod API key found" about a file it read without error that contained a valid key. This adds a file-based fallback: when the profile lookup finds no usable api_key, flash parses the config file directly (via tomllib) and returns the top-level apikey (apiurl stays ignored). Precedence is unchanged: env var > [default].api_key > runpodctl top-level apikey.

Fixes #363
Internal: CON-1228

What changed

• src/runpod_flash/core/credentials.py: get_api_key() now falls back to a new _get_runpodctl_api_key() helper that parses the credential file with tomllib and returns the top-level apikey when the [default] profile has no usable key. Missing file, malformed TOML, and blank/non-string keys all yield None as before.
• tests/unit/test_credentials.py: new tests — runpodctl-format config (top-level apikey only) yields the key; [default] profile takes precedence over the top-level key; blank top-level apikey is ignored; no credentials file still raises RunpodAPIKeyError via validate_api_key().

How verified

• pytest tests/unit/test_credentials.py tests/unit/test_credential_migration.py — 33 passed.
• pytest on all credential/login/API-key-related unit files (7 files) — 104 passed.
• ruff check and ruff format --check on both touched files — clean.
• mypy full run: no net-new errors vs main (261 pre-existing errors; the 2 attributed to credentials.py are pre-existing patterns — tomllib stub and Any return — identical before and after).

The write path (save_api_key) already preserved runpodctl's top-level keys; this PR only fixes the read path.