Add slog.Logger support for feature flag eval failure observability

Adds an optional Logger *slog.Logger field to Config. When set, the package emits a structured Warn-level log entry on every evaluation failure, including the flag key and error message.

This closes the observability acceptance criterion of team-tasks #4382: "Timeout events are observable via logs or metrics."

Changes

• Config.Logger *slog.Logger — optional, fully godoc'd
• Client.logger — wired from Config in NewWithConfig
• evaluate helper — calls c.logger.WarnContext(...) when err != nil
• TestBooleanValueDetails_LogsOnFailure — asserts the warning is emitted for a missing flag

Test plan

• go test ./featureflag/... -v -run TestBooleanValueDetails_LogsOnFailure passes
• Existing tests unchanged

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Stack (merge in order)

#	MR	Status
1	!485 (merged) — Add httptest slow-server timeout test	merge into master first
2	!486 (merged) — Add slog.Logger observability ← you are here	retarget to master after !485 (merged) merges
3	!487 (merged) — Document error propagation	retarget to master after !486 (merged) merges