Add managed variables

[dmontagu]

Summary

Adds a managed variables feature to Logfire, enabling dynamic configuration management with feature flags, A/B testing, and targeted rollouts. Variables can be resolved from local in-memory configuration or from the remote Logfire API with background polling and SSE-based real-time updates.

This is a replacement of #1548 to reduce review noise.

Key additions

• logfire/variables/ module — Variable class, VariableProvider ABC, local and remote providers, config models (rollout, conditions, variants)
• Logfire.var() — creates managed variables with typed defaults, optional resolve functions, and Pydantic validation
• variables_push / variables_validate / variables_push_types — CLI-friendly methods for syncing variable definitions to a provider
• Targeting & rollout — deterministic variant selection via targeting keys (user ID, trace ID), weighted rollouts, condition-based overrides (attribute matching, regex, presence checks)
• Context managers — variable.override(), variable.use_variant(), targeting_context() for scoped variable behavior
• Integration with logfire.configure() — VariablesOptions dataclass, provider lifecycle tied to config/shutdown

Unresolved review comments from #1548 to revisit

These are comments that were noted but not actioned yet in this PR:

1. Verb/naming inconsistency — Alex noted the mismatch between "get" vs "pull" and "sync" vs "push" in method names. Worth a naming review pass.
2. Method names sound generic — Alex suggested the variables_ prefix approach (already done) but also noted names could still be confused with logfire.configure options.
3. VariantKey/VariableName as NewTypes — Alex asked why not. David responded concerns about runtime overhead and deepening Pydantic dependency; changed to Pydantic models per Marcelo's suggestion. May revisit.
4. logfire/variables/config.py unused code — Alex asked "is this used?" about some config code (around line 473). Worth auditing.
5. Stale docs reference — Alex noted docs/reference/advanced/managed-variables.md:875 references a method that
   doesn't seem to exist. Docs may need a cleanup pass.
6. Provider start lifecycle — Devin flagged that start(None) in _initialize() followed by start(logfire_instance) in configure() is fragile. Currently works because _logfire is set before the _started early-return, but the two-phase start is worth reviewing.
7. PEP 747 (TypeForm) — Viicos suggested we could drop the sequence-of-types overload once PEP 747 lands, allowing type: str | int directly.
8. TypeAdapter deferred build — Viicos suggested using defer_build config for variables. David opted for eager build for now to avoid forward-ref issues; can revisit if build time is a problem.

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