define types for commonly-used `error_code` strings

[plebhash]

this was discussed on Sv2 Working Group call of April 30th 2026 and proposed by @coleFD

this is tied to stratum-mining/sv2-spec#194 and stratum-mining/sv2-spec#165

the core concepts here are:

• the spec allows for implementations to take automated actions on error_code strings
• the spec does not force implementations to use specific error_code strings, only provides suggestions for common scenarios

so we end up in a situation where different implementations could be aiming for the same error_code string, but still be subject to small typo or encoding issues

while that is hard to avoid without major spec changes, SRI can greatly reduce the room for such scenarios by providing types for commonly-used error_code strings

these should be provided on their respective message-crates:

• common_messages_sv2
• mining_sv2
• job_declaration_sv2
• template_distribution_sv2

[plebhash]

we should add constants for all error_code strings that are currently used on sv2-apps

[plebhash]

tbh, I don't think we need to wait for stratum-mining/sv2-spec#194 to be merged

as is, SRI is already diverging from the spec sentence discussed in stratum-mining/sv2-spec#165, so the improvement proposed here will not change anything with regards to that

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moreover, after considering @GitGab19 's suggestion #2149 (comment) I think addressing this first would be a good pre-requisite for a more concise approach in #2149

I'll start tackling this now