Title: [Bug]: Duplicate-email recipients collapse onto one recipient when resolving PDF placeholder tags ({{signature, rN}})

Issue Description

When creating an envelope via POST /api/v2/envelope/create with an explicit recipients array where two or more recipients share the same email (but different signingOrder/name), every PDF placeholder tag ({{signature, rN}}, {{date, rN}}, etc.) resolves to the same recipient, regardless of which rN index the tag actually references. All fields for every tagged recipient end up bound to a single recipientId — in practice, whichever recipient row shares that email and got matched first.

This makes it impossible to build a single-document flow where one person needs to sign in more than one place/step (e.g. the same signer countersigning at steps 1, 4 and 6) using placeholder-tagged PDFs, without giving that signer distinct alias emails.

Steps to Reproduce

1. Have a PDF with placeholder tags for at least 2 recipients, e.g. {{signature, r1}} and {{signature, r2}} at different positions.
2. POST /api/v2/envelope/create (multipart) with:
   json复制

   {
     "type": "DOCUMENT",
     "title": "repro",
     "recipients": [
       { "email": "[email protected]", "name": "A", "role": "SIGNER", "signingOrder": 1 },
       { "email": "[email protected]", "name": "B", "role": "SIGNER", "signingOrder": 2 }
     ],
     "meta": { "signingOrder": "SEQUENTIAL" }
   }

   (attach the tagged PDF as files)
3. GET /api/v2/envelope/{id} and inspect fields[].recipientId.

Expected Behavior

Two fields are created: one bound to the recipient with signingOrder: 1, the other to signingOrder: 2

matching the r1/r2 tags, exactly as happens when the two recipients have distinct emails.

Current Behavior

Both fields are created bound to the same recipientId. The second recipient gets no fields at all.

Screenshots (optional)

No response

Operating System [e.g., Windows 10]

N/A (self-hosted Docker deployment, Linux host) reproduced purely via REST API, not a client-UI issue.

Browser [e.g., Chrome, Firefox]

N/A — reproduced via REST API.

Version [e.g., 2.13.0]

2.15.0

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• I have searched the existing issues to make sure this is not a duplicate.
• I have provided steps to reproduce the issue.
• I have included relevant environment information.
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