fix: keep every factor of coupled infection rate expressions

[AdishAssain]

Bug

In a multi-patch or grouped model, an infection transition whose rate expression mentioned the parameter beta was replaced by beta * S * lambda. Every other factor in the expression was discarded, and the outcome depended on the parameter's name.

Two-patch SIR template, final I in patch A after 60 days:

Expression	Final I
S -> I: beta	0.2113
S -> I: beta * 0.5	0.2113, identical
S -> I: b * 0.5, same value	0.0235

An intervention term such as beta * (1 - efficacy * coverage) therefore ran at full beta with no warning. No test covered a multiplied beta in a coupled model.

Fix

_adjust_infection_rate now multiplies the local flow by the infectious pressure for every infection transition under mixing, which is the rule docs/rate-multiplication.md already documents. The special case, its regex on the parameter name, and the parameters that only served it are removed.

Behaviour change to note

A coupled infection expression that names I or E, for example beta * I / N, is now honoured as written, so the flow carries the local prevalence and the infectious pressure. Before, that double counting was avoided only when the parameter happened to be named beta. The documentation already says not to add a second infectious proportion to a coupled template; the model now logs a warning at construction when it finds one.

The S > 0 guard that existed on one of the two old branches is gone. The local flow already carries S, so a zero S gives a zero flow, and a negative S from solver overshoot gives a restoring flow rather than a frozen negative state.

Tests

• Coupled flow equals local flow times lambda for the documented template.
• A constant factor in the expression scales the coupled flow.
• The parameter name does not change the result.
• Recovery flow is not scaled by the infectious pressure.
• Warning is logged for an expression naming I, and not for a per-capita rate.

ruff check, ruff format --check, and pytest with 183 passed are clean.

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Reviewing files that changed from the base of the PR and between e947e4b and eee0749.

📒 Files selected for processing (3)

• CHANGELOG.md
• src/patchsim/core/model.py
• tests/test_model.py

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