fix(loop): preserve unavailable request-budget compatibility

[Brian Krabach (bkrabach)]

What changed

• Handle an initial request_budget result of None as unavailable, preserving the established normal-dispatch path.
• Once a concrete budget has required a context rebuild or output-reserve probe, fail locally if the budget capability disappears or returns None; do not treat uncertainty as a fit or send an unchecked SDK request.
• Add consumer coverage for cold and calibrated/warm behavior, including completed fake-SDK calibration before the existing warm-compaction protections.

Why

A provider can lack a trustworthy cold budget before it has calibration data. This consumer must keep that compatibility case distinct from a later loss of a capability already used to enforce a concrete budget. The latter remains fail-closed, without dropping input.

Compatibility

• Providers without request_budget, and an initial unavailable budget, keep normal dispatch behavior.
• A later unavailable result during an active budget-enforcement sequence now raises locally before SDK dispatch.
• No input is silently omitted; this change adds no configuration or public API surface.

Verification

• Loop non-live suite: 320 passed; 1 optional cross-module collection skipped in the SDK-less environment.
• Companion provider non-live suite: 984 passed; 2 deselected.
• Cross-module fake-SDK checks: 2 existing checks and 3 new mechanism cases passed, covering cold uncertainty, calibration, low output-cap probing, one-shot warning behavior, and warm-history protection.
• Source review passed; no paid model calls were made.

Breaking changes

None. The only changed failure behavior is intentional: a budget capability that becomes unavailable after returning a concrete decision fails closed before dispatch.

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