fix(sandbox): validate required textual fields

[altrudev]

Closes #289.

What

The sandbox adapter's public dataclasses accepted several runtime-supplied values directly into regex or membership checks before establishing that they were strings. Malformed non-string inputs could therefore escape as bare TypeError instead of the adapter's field-specific ValueError refusal.

This change establishes the primitive boundary for:

• SandboxAttestation.platform;
• SandboxAttestation.measurement;
• SandboxSessionResult.sandbox_id;
• SandboxSessionResult.image_digest.

The existing accepted syntax and semantics are unchanged.

Regression coverage

Focused tests exercise representative non-string values (True, integer 1, array [1], object {"x": 1}) across all four required textual fields, plus valid textual controls.

Scope

No schema change, no wire-format change, no URL/digest syntax widening, and no change to optional sandbox fields.

AI-assistance disclosure: ChatGPT assisted with source triage, adversarial-case design, implementation drafting, duplicate/ownership review, and diff review. altrudev reviewed the bounded claim and remains responsible for the contribution.

[github-actions]

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[lywinged]

Approved on 5b4da40 over fc38496. Reproduced on main: SandboxAttestation(measurement=...) with true, 1, [1], {"x": 1} or null escapes as a bare TypeError from the regex, and platform set to [1] or {"x": 1} escapes as an unhashable-type TypeError from the set lookup; hashable non-strings on platform already produced the field's ValueError. On this head all ten inputs produce the field-specific ValueError, the positive control passes, and the PR's test file has 14 of 17 cases failing against main and none here. One step over, outside this PR: the optional fields have no boundary at all. rim_uri=1 constructs, and is refused only by the pydantic RuntimeInfo inside build_trust_record(), which is the later, generic error the comment above _SUBJECT_RE says these dataclass checks exist to pre-empt. Same class, for a follow-up. agt.py has no equivalent pattern. Suite 1,184 passed, 1 skipped; ruff, mypy and the dash check clean.

Tool-assisted: the runs and this write-up.