Same token policy buy support

[extrawurst]

Description

Adds buy-side support for orders whose buy token equals the sell token (part of
#3963) via a new SameTokensPolicy::Allow variant. AllowSell only permitted
sell orders, so buy same-token quotes were rejected at validation.

This only adds the capability and test coverage — the default policy stays
Disallow and shipped configs keep AllowSell, so there is no production
behavior change until a config opts into allow.

Changes

• Add the SameTokensPolicy::Allow variant and handle it in order validation.
• Add a buy-side e2e test settling a same-token (WETH→ETH) order end to end.
• Add unit coverage for Allow validation and for the same-token out_amount
  correction in the trade verifier, which buy quotes can now reach.

How to test

cargo nextest run -p shared -p configs -p price-estimation
cargo nextest run -p e2e local_node --run-ignored ignored-only --test-threads 1 --failure-output final

[gemini-code-assist]

Code Review

This pull request implements buy-side support for same sell and buy token orders under the SameTokensPolicy::Allow policy. It updates the order validation logic, refactors the trade verifier's post-processing to support this verification, and adds corresponding unit and end-to-end tests. The feedback highlights a misleading comment in the newly added end-to-end test that describes a successful quote submission as an expected failure point, which should be updated or removed.

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[squadgazzz]

LGTM, good tests 👍

[MartinquaXD]

Tests look good. So in practice no logic change was needed. We just needed to lift the limitation that buy orders are not supported.