fix: Convert datetime attributes to ISO-8601 strings

[kinyoklion]

Evaluation context attributes are now converted to values the LaunchDarkly context model can represent, instead of being passed through as-is.

• A datetime attribute previously reached the LaunchDarkly context unchanged, so the context could not be serialized: json.dumps on it raises TypeError: Object of type datetime is not JSON serializable, which breaks analytics event delivery for that context.
• datetime values now become their ISO-8601 representation in UTC, which is the format the LaunchDarkly date targeting operators accept. A naive datetime is treated as UTC.
• The conversion recurses into lists and dictionaries, so a nested datetime is converted too.
• A value of a type which cannot be represented becomes None and logs an error, rather than producing an unusable context.

This aligns the Python provider with the Java, .NET, and JavaScript providers, which already convert their SDK's date values, and with the date or instant row of the OpenFeature provider behavior spec's value conversion table.

Found during the weekly OpenFeature provider audit.

Implementation details

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

None.

Describe the solution you've provided

_to_ld_value in the context converter handles the OpenFeature value types: scalars pass through, a datetime is converted with astimezone(timezone.utc).isoformat() (with the +00:00 offset written as Z), mappings and sequences are converted item by item, and anything else becomes None with an error log. Custom attributes are set through it; name, anonymous, and privateAttributes keep their existing type-specific handling.

datetime is one of the types the OpenFeature Python SDK's EvaluationContextAttribute alias allows, so an application following the OpenFeature specification can supply one.

Describe alternatives you've considered

Rejecting a datetime attribute and logging an error: that loses targeting information the application intended to send, and date targeting operators exist precisely for this case.

Treating a naive datetime as local time: the local zone of a server is incidental, so the same context would convert differently depending on where it ran.

Additional context

Testing: make lint (mypy) and make test (86 tests). The new tests cover an aware datetime, a non-UTC offset, a naive datetime, a datetime nested inside a dictionary and list, JSON serializability of the resulting context, and an unconvertible value.

The README's OpenFeature specific considerations section documents the conversion.

Link to Devin session: https://app.devin.ai/sessions/fe1eb757fe694ef79f3d09f6307d4b47
Open in Devin Desktop: https://app.devin.ai/desktop/session/fe1eb757fe694ef79f3d09f6307d4b47?variant=devin
Requested by: @kinyoklion

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