Use full_relay_url in sender GET request

[spacebear21]

This fixes an issue with directories other than the GATEWAY_ORIGIN specified by the OHTTP relay. Previously, attempting to use a different directory using the gateway opt-in mechanism resulted in an error due to the ohttp relay routing the sender request to the wrong directory:

[payjoin-cli/src/app/v2/mod.rs:499:13] &response = Response {                                                                                                                                                             url: "https://ohttp.cakewallet.com/",                                                                                                                                                                                 status: 400,
    headers: {                                                                                                                                                                                                                "server": "nginx/1.27.4",                                                                                                                                                                                             "date": "Tue, 21 Oct 2025 00:27:30 GMT",                                                                                                                                                                              "content-type": "application/problem+json",                                                                                                                                                                           "content-length": "103",
        "connection": "keep-alive",
        "access-control-allow-origin": "*",
    },
}
Transient error: The receiver sent an invalid response: v2 encapsulation error: Directory response error: Unexpected response size 103, expected 8192 bytes

The directory URL must be included in the path but was previously only included in POST requests (introduced in 8906c76). This includes it in GET requests as well.

I used Sonnet 4.5 for troubleshooting.

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[coveralls]

Pull Request Test Coverage Report for Build 18670427278

Details

• 25 of 25 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 83.743%

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Totals
Change from base Build 18413003453:	0.006%
Covered Lines:	8953
Relevant Lines:	10691

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💛 - Coveralls

[DanGould]

ACK f724e42

Great find. That directory 1-1 GATEWAY_URL backwards compatibility is gonna need to get removed once Bull Bitcoin Mobile and Cake upgrade. I do wonder if it makes more sense to enforce in the Request::new_v2 rather than the fn full_relay_url