test: fill SDK coverage gaps

[somus]

This pull request adds new test cases across several SDK packages to improve coverage of previously untested functionality. The new tests cover:

• Core helpers: canonicalization of identity strings, stable UUID derivation with seed-part boundaries, and classification of credential key names, safe placeholders, and opaque token values.
• Redaction configuration: validation of custom regex patterns, ensuring that assertSafeRegexSource correctly rejects unsupported constructs (backreferences, lookarounds, unsafe quantifiers) while allowing escaped syntax.
• Redaction transform: entropy-based redaction behavior when explicitly enabled, and proper skipping of allowlisted secrets, opaque tokens, and already-redacted placeholders.
• Render model: transcript summary and filtering logic aligned with ported viewer oracle cases, including tool grouping, semantic call‑id handling, and orphan result detection.
• Sessions: discovery skipping of source references without paths, surfacing of health warnings, and propagation of reconcile passthrough warnings during session loading.
• Store paths: resolution of the store root from explicit arguments, environment variables, or HOME, plus validation of content‑hash inputs.

All changes are test-only additions and do not modify production code.

[kody-ai]

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[ghost]

Ready to review this PR? Stage has broken it down into 5 individual chapters for you:

	Title
1	Test core identity and credential helpers
2	Add redaction entropy and regex-safety tests
3	Test store path resolution and environment fallbacks
4	Expand session discovery and reconciliation coverage
5	Port render-model oracle to committed tests

_{Chapters generated by Stage for commit 3edb7dc on Jun 16, 2026 3:39am UTC.}