fix(weave): preserve typed message fields during PII redaction

[ash0ts]

Why

I hit this while testing a private HiveMind chat review mirror with redact_pii=True.

The Conversation SDK currently serializes each typed Message, recursively sends every string through Presidio, and then validates the result back into the typed model. That means a false positive on a protocol value can corrupt the payload before any span is emitted. In the concrete failure, Presidio classified the literal "uri" discriminator from UriPart.type as a person name, changed it to "<PERSON>", and Pydantic rejected the message union.

This makes URI-backed conversation content impossible to log with PII redaction enabled, even when the URI itself contains no sensitive text.

What changed

• Walk typed message parts directly instead of redacting a dumped protocol object.
• Preserve roles, union discriminators, IDs, MIME/modality metadata, blobs, file references, and URIs byte-for-byte.
• Continue using the same configured Presidio engines for user-authored text:
  • flat message content;
  • text and reasoning parts;
  • tool-call arguments; and
  • tool-call responses.
• Keep configured field-name redaction behavior for those textual payload fields.
• Add a regression where the fake analyzer deliberately marks the exact string "uri" as PII while also redacting an email in adjacent text. The resulting span must retain a valid UriPart and contain the redacted email.

Validation

• 23 passed in tests/conversation/test_conversation_settings.py
• Ruff lint passed
• Ruff formatting check passed
• git diff --check passed

This is a small companion fix for the review-mirror work in wandb/hivemind#8. It is independent of the proposed atomic historical-turn API and does not change OTLP delivery or idempotency semantics.

[codecov]

Codecov Report

❌ Patch coverage is 67.85714% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
weave/utils/pii_redaction.py	67.85%	6 Missing and 3 partials ⚠️

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[ash0ts]

Post-rebase validation

I cleaned this draft onto current master; it now contains exactly one commit and changes three files. The current branch result is 27 passing conversation-settings tests, plus clean Ruff lint, Ruff format, and git diff --check.

The HiveMind review mirror consumes the same fix on its combined pinned companion commit because it also needs the separate no-create initialization capability.