feat(inference): declare how a context window was determined

[flyworker]

Closes the request in #128.

The agent already distinguishes a window read from the backend's max_model_len from one asserted in models.json, but sent only the number. A receiver could not tell them apart, and they are not equally trustworthy:

• a detection is self-verifying — vLLM and SGLang refuse to start when --max-model-len exceeds what the KV cache can hold, so the value is backed by memory that demonstrably exists;
• an override is an assertion nothing on the agent side can check.

That matters once share of traffic is weighted by the declared window, because the override path is the one carrying the incentive.

What is sent

context_source now travels with the value, on both the heartbeat metadata and the schema-1 declaration:

{"model_id": "...", "context_length": 32768, "context_source": "detected"}
{"model_id": "...", "context_length": 128000, "context_source": "override"}
{"model_id": "...", "context_length": 32768}

Absent means unknown, by construction

As requested — an agent that does not send the field reads exactly as it did before, so this ships without a coordinated rollout.

unknown and pending are deliberately not sent explicitly: that is what an absent field already conveys, and emitting them would add a value every receiver has to special-case. A source is only ever attached to a window that was actually determined, so a length of 0 never carries one.

The tests assert the serialised form, not just the helper, because an empty string is a present field and would defeat the point.

Also

The provider function now returns the window and its provenance together rather than a bare int, so the two cannot drift apart at any of the three declaration sites.

Verified

4 new tests: the mapping for each source, the JSON genuinely omitting the key when undetermined, heartbeat metadata carrying provenance while an undetermined model is not declared at all, and the same for the schema-1 declaration including its serialised form. Existing context tests updated to the richer type.

8/8 packages, go vet and gofmt clean. Documented in docs/configuration.md.