feat!: replace Stainless-hosted code execution with self-hosted remote sandbox

BREAKING CHANGE: in @coingecko/coingecko-mcp, the codeExecutionMode value
'stainless-sandbox' (previously the default) and docsSearchMode 'stainless-api'
are removed, and the CODE_MODE_ENDPOINT_URL environment variable is no longer
read. Remote execution now uses codeExecutionMode 'remote-sandbox' with the new
sandboxEndpoint and sandboxFetcher options; the default execution mode is now
'local'. These changes first shipped in 6.0.2, which was mislabeled as a patch —
this release corrects the version number. @coingecko/coingecko-typescript (the
SDK) has no breaking changes and shares this major version only through
lockstep versioning.

Release-As: 7.0.0

Author: zacharygoh