Add --remote-auth-scope-param-name for non-standard OAuth scope parameters

[gmogmzGithub]

Summary

• Adds --remote-auth-scope-param-name CLI flag to override the query parameter name used for scopes in the OAuth authorization URL
• When set (e.g., user_scope), scopes are sent under the custom parameter name and the standard scope= is cleared
• oauth2Config.Scopes is preserved so token refresh continues to work correctly

Motivation

Slack's OAuth v2 requires user-token scopes in user_scope= instead of the standard scope= parameter. When ToolHive connects to https://mcp.slack.com/mcp, its OAuth discovery correctly finds the v2_user/authorize endpoint and the 15 supported scopes, but Go's oauth2 library always places scopes in scope=. Slack rejects this with invalid_scope.

This flag closes the gap so ToolHive can authenticate with providers that use non-standard scope parameter names.

Example usage for Slack MCP

thv run https://mcp.slack.com/mcp \
  --name slack \
  --transport streamable-http \
  --remote-auth-client-id 1601185624273.8899143856786 \
  --remote-auth-callback-port 3118 \
  --remote-auth-authorize-url "https://slack.com/oauth/v2_user/authorize" \
  --remote-auth-token-url "https://slack.com/api/oauth.v2.user.access" \
  --remote-auth-scope-param-name user_scope

Changes

File	Change
pkg/auth/oauth/flow.go	ScopeParamName field on Config; buildAuthURL override logic
pkg/auth/oauth/manual.go	Pass-through scopeParamName parameter
pkg/auth/discovery/discovery.go	ScopeParamName on OAuthFlowConfig
pkg/auth/remote/config.go	ScopeParamName field with JSON/YAML tags
pkg/auth/remote/handler.go	Wire ScopeParamName to flow config
cmd/thv/app/auth_flags.go	New --remote-auth-scope-param-name flag
cmd/thv/app/run_flags.go	Wire through both config builders
cmd/thv/app/proxy.go	Wire through both flow configs

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.62%. Comparing base (a388093) to head (141b5cf).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/auth/remote/handler.go	0.00%	2 Missing ⚠️
pkg/auth/discovery/discovery.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4712      +/-   ##
==========================================
+ Coverage   68.60%   68.62%   +0.02%     
==========================================
  Files         517      517              
  Lines       54631    54639       +8     
==========================================
+ Hits        37480    37498      +18     
+ Misses      14265    14246      -19     
- Partials     2886     2895       +9     

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