Give migration directions for the removed gov.dwp.carer_premium.couple parameter (generalise add_removed_parameter_aliases) #2137

Problem

#2001 (merged as 3518331, released in 2.117.0) removed gov.dwp.carer_premium.couple. It held the single rate, and the law pays gov.dwp.carer_premium.single once for each qualifying claimant or partner. A saved policy or reform that still sets couple now fails with a bare lookup error:

ValueError: Could not find the parameter gov.dwp.carer_premium.couple (failed at couple).

Main already has a mechanism that fails with migration directions instead, REMOVED_PARAMETERS in policyengine_uk/utils/parameters.py. It is used for gov.dwp.state_pension.age.male and .female. But add_removed_parameter_aliases installs every alias under parameters.gov.dwp.state_pension.age. Adding the key gov.dwp.carer_premium.couple to the dict would therefore create gov.dwp.state_pension.age.couple and leave the carer premium path unchanged.

Proposed fix

1. In add_removed_parameter_aliases, resolve each removed path's parent node from the path itself, instead of using the fixed State Pension age node.
2. Then add "gov.dwp.carer_premium.couple" to REMOVED_PARAMETERS, with directions like: "The carer premium is now paid per qualifying claimant or partner; change gov.dwp.carer_premium.single (the per-person amount)."
3. Test that Reform.from_dict({"gov.dwp.carer_premium.couple": ...}) and a scenario naming the path both raise the new message. Also test that no alias appears under gov.dwp.state_pension.age.

Found by the independent review of #2001 after its merge with main (finding 5). It is not a correctness issue, because the reform already fails loudly.

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