fix: Validate and sanitize Codecov token before use in Authorization header

[sentry]

This PR addresses an InvalidHeader error that occurred when the CODECOV_TOKEN environment variable (or -t flag) contained a YAML-structured multi-line string (e.g., codecov:\n token: <UUID>) instead of the plain UUID token.

The requests library, used for HTTP communication, rejects header values containing newline or carriage-return characters. Previously, the CLI would pass the malformed token directly, leading to a crash.

To fix this, a new helper function _validate_token has been introduced in codecov_cli/helpers/request.py. This function:

1. Converts the token to a string and strips leading/trailing whitespace.
2. Checks for the presence of newline (\n) or carriage-return (\r) characters within the token.
3. If such characters are found, it raises a click.ClickException with a clear, actionable error message, guiding the user to provide only the plain UUID token.

This validation is applied in both get_token_header_or_fail and get_token_header to ensure all Authorization headers are properly formed, preventing crashes and improving user experience for misconfigured tokens.

Fixes CLI-GT

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[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1726 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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