fix(agent-sre): preserve alert fingerprint field boundaries - #4208

Description

Distinct alerts can serialize to the same deduplication input. For example, (agent_id='agent|title=worker', title='Breach') and (agent_id='agent', title='worker|title=Breach') both become agent_id=agent|title=worker|title=Breach; the second alert is incorrectly suppressed.

Encode the field/value pairs as JSON before hashing. The regression exercises delivery of both alerts and continued suppression of actual duplicates. Fingerprint digests change; the built-in deduplication state is in memory, and no persisted-key migration is added.

Validation

The delivery regression fails before the fix. Alert and deduplication tests: 67 passed. Full package run: 1,495 passed, 3 skipped, 1 known baseline failure (the separate Windows benchmark clock bug).

All regressions were reproduced on origin/main 406ddd4d before the production change. Whitespace checks pass. Ruff passes on the changed files. Changed-line spelling passes with the allowance for the existing API identifier committed on the new test line.

Scope: the existing Agent-SRE source tree, which remains exercised by repository CI. Combined validation of these six independent SRE fixes: 1,505 passed, 3 skipped on Windows/Python 3.12.

Commit d443efd adds a spelling allowance for the existing AlertDeduplicator identifier on the new test line. The changed-line artifact passes the CI-pinned cspell 8.17.3 with the repository configuration, and the alert deduplication test file passes again (34 tests).

Type of Change

• Bug fix (non-breaking correction of existing behavior)

Package(s) Affected

• agent-sre

Checklist

• Changed code follows the project style checks; the exact validation scope is recorded above.
• Regression tests or existing package tests verify the change, as detailed above.
• The independent full package run has no failures: one pre-existing Windows benchmark failure remains, as documented above. Focused tests pass; the combined run with fix(agent-sre): measure benchmark duration with a monotonic clock #4206 passes (1,505 passed, 3 skipped).
• Documentation is updated where needed; the behavior and compatibility implications are described above.
• The Microsoft CLA check has passed for this contribution.

Attribution & Prior Art

• Related prior contributions are credited below; this PR introduces no unattributed code from external projects.
• External design influences, where applicable, are identified below.

Prior art / related projects: Uses the existing Agent-SRE implementation and standard library operations. No code was copied or adapted from an external project.

AI Assistance

I reviewed the specific changes in this diff before this PR was opened.

Codex assisted with implementation, regression tests, and local validation under my direction.

• I reviewed this diff before submission; its behavior, rationale, and tradeoffs are documented above.
• Tests and verification appropriate to this change were run, with limitations disclosed above.
• Submission followed my direction and my review of these specific changes.
• No AI-generated reviews of other contributors' PRs were posted as part of this contribution.

IP, Patents, and Licensing

• This change uses the existing project implementation and ordinary software techniques; no proprietary or patent-specific technique is introduced.
• Understanding or using this change requires no NDA or separate licensing agreement.
• This contribution remains under the repository's MIT License and introduces no third-party source code or additional license terms.

Related Issues

Newly reproduced defect; the reproduction and regression coverage are described above.