fix(elements): name a system alert that blocks the screen read, at once

[leeguooooo]

Why: On a real phone, Xiaohongshu's system "Allow Paste" prompt made /source fail 109 times. Each /agent/elements read then hit the 35 s budget and timed out with no hint of why; WDA also dropped once. Reported by the parallel iphone-use session.

What:

• The source-read loop probes /alert/text (about 0.3 s) on the first failure and once more after about 3 s.
• If an alert is up, the read returns 409 {error: alert_blocking, alert: {text, buttons}, hint} immediately.
• A read that still fails or times out also carries the alert block when one is up.
• docs/agent-reference.md documents this.

Tests:

• New tests/source_alert.rs: the alert is named in under 5 s (0.07 s in the test), with its buttons.
• The existing revocation test's mock now answers the probe with "no alert".
• cargo test -p server and -p iphone-use-mcp pass.

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Reviewing files that changed from the base of the PR and between 71c0c9e and b29c576.

📒 Files selected for processing (4)

• crates/server/src/http.rs
• crates/server/tests/http_auth.rs
• crates/server/tests/source_alert.rs
• docs/agent-reference.md

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