Fix grammatical errors in e2e scenarios documentation

[coderabbitai]

Description

Three scenario descriptions in docs/e2e_scenarios.md contain a grammatical error where "in unreachable" should be "is unreachable".

Affected lines:

• Line 106: models endpoint
• Line 108: shields endpoint
• Line 110: tools endpoint

The phrase "when llama-stack in unreachable" should be corrected to "when llama-stack is unreachable" in all three instances.

References

• Pull Request: Updated e2e scenarios #1077
• Review Comment: Updated e2e scenarios #1077 (comment)
• Requested by: @tisnik