fix(codex): align codex-pro context_length with client catalog max_context_window

[jcardama]

Problem

f1d6988 refreshed codex_client_models.json, moving max_context_window for the GPT-5.6 family from 921000 to 872000 and adding gpt-6-astra at 872000. The matching context_length values in the codex-pro section of models.json were not moved, so the two files now disagree:

model	models.json codex-pro	codex_client_models.json max_context_window
gpt-6-astra	272000	872000
gpt-5.6-sol	921000	872000
gpt-5.6-terra	921000	872000
gpt-5.6-luna	921000	872000

Astra is the odd one out in a second way. Its 272000 is the context_window value, while its three neighbours use max_context_window semantics, so Astra currently advertises a smaller context than Sol despite being the higher-priority model.

Precedent

34a122f, "Update GPT-5.6 context length to 921000 for Codex Pro and client models", changed context_length in models.json and max_context_window in codex_client_models.json in the same commit, to the same number. This PR applies that same pairing to the value f1d6988 just landed.

Evidence

GET https://chatgpt.com/backend-api/codex/models?client_version=0.153.2 on a Pro account, captured under both originators:

slug	context_window	max_context_window
gpt-6-astra	272000	872000
gpt-5.6-sol	272000	872000
gpt-5.6-terra	272000	872000
gpt-5.6-luna	272000	872000
gpt-5.5	272000	272000

codex-tui and codex_cli_rs agree on every row. gpt-5.5 is included as a control: it reports 272000 for both fields and is already stored as 272000, so it needs no change and is left untouched.

Scope

Four context_length values in the codex-pro section, set to 872000. Nothing else is modified.

codex-team and codex-plus still carry 372000 for the 5.6 family and 272000 for Astra. I have left those alone deliberately: I only have a Pro credential, so I cannot observe what those plans actually return, and I would rather not change a number I cannot verify. If 372000 is a real per-plan limit then Astra's 272000 in those two sections is probably also wrong, but that needs someone on a Plus or Team account to confirm.