fix: empty/whitespace credentials should raise MissingCredentialsError

[Kamilbenkirane]

Summary

Closes #184

• get_credentials() now rejects empty/whitespace SecretStr values with MissingCredentialsError instead of passing them through to produce invalid HTTP headers
• has_credential() now returns False for empty/whitespace credentials, which also fixes list_available_providers()
• Inverted test_whitespace_credentials to assert the new behavior

Test plan

• test_whitespace_credentials_are_treated_as_missing passes for "", " ", "\t\n"
• All 478 unit tests pass
• All pre-commit hooks pass (ruff, mypy, bandit)

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[Kamilbenkirane]

Code review

Found 3 issues:

1. override_key whitespace bypass: create_client(api_key=" ") flows into get_credentials(override_key=" "). Since " " is truthy in Python, it bypasses the new whitespace guard at line 119 and returns SecretStr(" ") directly, producing a malformed Authorization: Bearer header. This is the same bug class the PR fixes for the env-var path, but the override_key path is left unguarded.

celeste-python/src/celeste/credentials.py

Lines 100 to 105 in c82f1fc

	"""
	if override_key:
	if isinstance(override_key, str):
	return SecretStr(override_key)
	return override_key

2. get_credentials docstring incomplete: The Raises section says "If provider requires credentials but none are configured" but the implementation now also raises for empty/whitespace-only values. Should mention the new condition, e.g. "or the configured value is empty or whitespace-only."

celeste-python/src/celeste/credentials.py

Lines 97 to 100 in c82f1fc

	Raises:
	MissingCredentialsError: If provider requires credentials but none are configured.
	"""

3. has_credential docstring incomplete: The one-liner "Check if a specific provider has credentials configured" doesn't reflect the behavior change -- whitespace/empty values are now treated as "not configured." A clarifying sentence would make the new semantics discoverable.

celeste-python/src/celeste/credentials.py

Lines 132 to 133 in c82f1fc

	def has_credential(self, provider: Provider) -> bool:
	"""Check if a specific provider has credentials configured."""

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