Support explicitly known-absent properties in local evaluation and /flags

[marandaneto]

Problem

SDK local evaluation needs to distinguish between:

1. a property key that is omitted because the caller supplied an incomplete property map, and
2. a property key that the caller explicitly knows is absent.

Today, omission can only be treated as unknown. For an is_set condition this means:

• a present key, including one whose value is null, evaluates as set;
• a missing key makes local evaluation inconclusive and leaves remote evaluation eligible;
• known absence cannot be represented.

Treating every missing key as unset would be unsafe because an incomplete property map could then produce an incorrect local result.

Context: PostHog/sdk-specs#49 (comment)

Required API changes

SDK evaluation APIs

Add an optional, backward-compatible way for callers to identify keys known to be absent, for example:

personPropertyKeysKnownAbsent
groupPropertyKeysKnownAbsent

/flags API

When local evaluation is inconclusive and falls back to remote evaluation, the SDK must be able to send the same known-absence information. Otherwise the server may use stored property values and return a result inconsistent with local evaluation.

Possible optional request fields:

{
  "person_property_keys_known_absent": ["plan"],
  "group_property_keys_known_absent": {
    "organization": ["subscription"]
  }
}

Expected outcome

• Callers can explicitly represent known-absent person and group properties.
• Local and remote evaluation apply the same absence semantics.
• Existing clients that do not send the new fields retain current behavior.
• The behavior and request schema are documented so SDKs can implement it consistently.

[marandaneto]

probably relates to #52747 and #29916 as well

[marandaneto]

cc @haacked