bench: improve context creation in ECDH benchmark #1748

1. 
   theStack commented at 9:34 pm on September 16, 2025: contributor

   Calling secp256k1_context_create with SECP256K1_FLAGS_TYPE_CONTEXT seems to be confusing and not strictly API-compliant, as the only allowed (non-deprecated) value is SECP256K1_CONTEXT_NONE, even if the former happens to map to the latter currently.

   Fix this by not dynamically creating a context in the first place and switch to using the static context, as it is sufficient for this benchmark and presumably matches what the “no capabilities” comment intended back then.

   Alternatives are:

   • keep the signing context and only fix the name, i.e. s/_FLAGS_TYPE_CONTEXT/_CONTEXT_NONE/
   • use secp256k1_context_static everywhere directly and get rid of the ctx field in the bench_ecdh_data struct (less flexible for future changes, deviates from other bench structures)

   Found while reviewing #1698, see #1698 (review).

2. bench: improve context creation in ECDH benchmark

   Calling `secp256k1_context_create` with `SECP256K1_FLAGS_TYPE_CONTEXT`
   seems to be not strictly API-compliant, as the only allowed
   (non-deprecated) value is `SECP256K1_CONTEXT_NONE`, even if the
   former happens to map to the latter currently.
   
   Fix this by not dynamically creating a context in the first place and
   switch to using the static context, as it is sufficient for this
   benchmark and presumably matches what the "no capabilities" comment
   intended back then.

   dfe284ed2d

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