feat: add create_subscription_registration_link MCP tool

[jating06]

Summary

• Adds create_subscription_registration_link MCP tool wrapping the Razorpay POST /v1/subscription_registration/auth_links API
• Registered as a write tool in the payment_links toolset
• Allows merchants to create auth links sent to customers (via SMS/email) to authorize recurring payment mandates (card, emandate, nach, etc.)
• Also fixes a pre-existing build failure in pkg/mcpgo/server.go caused by the mcp-go v0.45.0 upgrade changing OnAfterCallToolFunc's result parameter type from *mcp.CallToolResult to any

Parameters

Parameter	Type	Required	Description
customer	object	✅	Customer details: name, email, contact
type	string	✅	Auth mechanism type ("link")
amount	number	✅	Amount in smallest currency subunit
currency	string	✅	ISO 4217 currency code (e.g. INR, MYR)
subscription_registration	object	✅	Mandate details: method, max_amount, expire_at, frequency
description	string	☐	Description of the registration link
receipt	string	☐	Internal receipt reference (max 40 chars)
email_notify	boolean	☐	Send email notification
sms_notify	boolean	☐	Send SMS notification
expire_by	number	☐	Unix timestamp when the link expires
notes	object	☐	Key-value metadata (max 15 pairs)

Test plan

• go test ./... passes (all 9 packages)
• No new long lines without //nolint:lll
• Tests cover: full success, required-params-only success, missing required param, wrong type, API error

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.18182% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.33%. Comparing base (64e5a74) to head (ea0873c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/razorpay/subscription_registration_links.go	98.05%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   97.32%   97.33%   +0.01%     
==========================================
  Files          34       35       +1     
  Lines        7370     7480     +110     
==========================================
+ Hits         7173     7281     +108     
- Misses        134      135       +1     
- Partials       63       64       +1     

Flag	Coverage Δ
unittests	97.33% <98.18%> (+0.01%)	⬆️

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