Legal UI - Max. Number of Shares as "0" makes correction fail

_SB: I added a second bug to this ticket. See below._

**Describe the bug in current situation**

1. Due to bug [# 7505](https://app.zenhub.com/workspaces/entity-5bf2f2164b5806bc2bf60531/issues/bcgov/entity/7505), the UI allowed the entry of "0" number of shares. This UI bug has now been fixed. This ticket is to add API validation code to report an error if the number of shares is "0" (and any other fix required so the filer doesn't fail).

2. Due to bug [# 7488](https://app.zenhub.com/workspaces/entity-5bf2f2164b5806bc2bf60531/issues/bcgov/entity/7488), the UI allowed the entry of a huge number of shares. This UI bug has now been fixed. This ticket is to add API validatio code to report an error if the number of shares is more than 15 digits (and any other fix required so the filer doesn't fail).

I made this a P2 since the Legal API is not yet open for others to use, and the only UIs to write to it will no longer save invalid Number of Shares.

**Link bug to the User Story**

**Impact of this bug**
Describe the impact, i.e. what the impact is, and number of users impacted.

**Chance of Occurring (high/medium/low/very low)**

**Pre Conditions: which Env, any pre-requesites or assumptions to execute steps?**

**Steps to Reproduce**
Steps to reproduce the behavior:
1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

**Actual/ observed behavior/ results**

**Expected behavior**
A clear and concise description of what you expected to happen. Use the gherking language.

**Screenshots/ Visual Reference/ Source**
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Start by tracing the Legal API handling and validation for Number of Shares; the issue does not name a file, test, or entry point. Reproduce the zero-share and over-15-digit cases, then verify that both are rejected with an error and that the filer does not fail unexpectedly.

Written by the indexing model from the issue text.