Keep an Alpha Vantage rate-limit notice retryable

[thorstenalpers]

Follow-up to #63.

Changes

• The negative cases in tests/IntegrationTests/AlphaVantageTests.cs use Assert.CatchAsync instead of Assert.ThrowsAsync. ThrowsAsync matches the exact type, so they fail since a refused request throws the derived FinanceNetAccessDeniedException. CI runs unit tests only and did not show this.
• The premium marker is now the full sentence "This is a premium endpoint". The rate-limit notice ends with "unlock all premium endpoints", so the shorter marker turned a temporary rate limit into a permanent, non-retried refusal.
• "rate limit" is recognised as a retryable limit next to the old "higher API call volume" wording. Without it, GetOverviewAsync deserialized the notice to an empty object and threw FinanceNetNoDataException after a single request.
• The rate-limit test now covers all four methods with a realistic notice, the invalid-key test asserts InnerException is null, and the XML summary on the private ThrowIfRejected became a one-line comment.

No public API change, Constants is internal.

Verification

• dotnet build Finance.NET.slnx --configuration Release: 0 warnings, 0 errors
• dotnet test tests/Tests.csproj --configuration Release --filter "TestCategory=Unit": 207 passed, 0 failed
• Mutation check: with the old "premium endpoint" marker the four RateLimit_IsStillRetried cases fail with FinanceNetAccessDeniedException.
• Not run: the live Alpha Vantage integration tests. The rate-limit notice text in the test is reproduced from memory, not captured from a live response.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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