test(browser): fail closed on oversized Agent Task observation

[seonghobae]

Partial implementation of #28, stacked on unchanged exact PR #105 head bc1d22d6c4848a173c55fdd18054574299488067.

Buyer/security gap

The pinned-Chromium controlled Agent Task measures its semantic-observation JSON byte size but only rejects the impossible zero-length case. Issue #28 requires oversized observation/text to be truncated or fail closed. Without an explicit byte ceiling at the executable browser-evidence boundary, later fixture growth or adapter expansion could silently turn an already bounded role/name snapshot into unbounded evidence.

Dependency

This Draft targets exact #105 head bc1d22d6c4848a173c55fdd18054574299488067 and must remain Draft while #105/#100 and their prerequisites remain active. No prerequisite check, review, status, artifact, coverage, or mergeability result transfers.

TDD state

Exact test-only head 71e78784332b2cbe34d3d704b6a03d0d69ff35c2 defines the missing executable bound before runner support exists. The intended contract requires:

• one explicit finite MAX_AGENT_TASK_SEMANTIC_OBSERVATION_BYTES ceiling;
• canonical compact UTF-8 JSON measurement;
• exact-limit acceptance and one-byte-over fail-closed rejection;
• rejection of empty and non-object observation values; and
• the real controlled Agent Task browser pass to use the bounded measurement helper rather than an unbounded direct len(json.dumps(...)) path.

Production-like runner support is intentionally absent at this head so CI can establish the realistic RED.

Truth boundary

This lane bounds only the controlled semantic-observation evidence emitted by the pinned-browser compatibility runner. It does not define the final product-wide semantic snapshot budget, truncate browser content, authenticate a browser adapter, authorize model context, expose raw HTML, add arbitrary JavaScript, or complete the end-to-end OriginWeave browser adapter. A later production adapter must apply its own reviewed structured-observation bounds at the real observation boundary.

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