Fix API payload truncation, add client-secret action input, and cap workflow run pagination

[Rishabh4275]

Motivation and Context

1. API payloads were silently truncated — A jsonReplacer intended for log sanitization was also used when serializing HTTP request bodies via JSON.stringify(payload, this.jsonReplacer). Any string field over 1,000 characters (file diffs, PR descriptions, commit details) was truncated to 100 characters + "... [truncated in logs]" in the
   actual API request, causing incomplete data to be sent to Purview.

2. client-secret required an environment variable instead of an action input — client-certificate was a proper action input (core.getInput), but client-secret was
   read from process.env['AZURE_CLIENT_SECRET']. This was inconsistent and required users to pass the secret via env: instead of with:.

3. Workflow run history check had no pagination cap — findLastProcessedCommitSha() paginated through all successful workflow runs with no upper bound. For repos with
   long histories this could result in excessive API calls, even though the query is already scoped to a single branch.

Description

Fix API payload truncation:

• Removed this.jsonReplacer from all 5 JSON.stringify() calls that serialize HTTP request bodies in PurviewClient (processContentAsync, processContent,
  uploadSignal, searchTenantProtectionScope, searchUserProtectionScope). These now use plain JSON.stringify(payload).
• Deleted the now-unused jsonReplacer method from PurviewClient. The logger's own jsonReplacer (in logger.ts) is unaffected and continues to handle log formatting
  separately.

Add client-secret action input:

• Added client-secret as a new input in action.yml, alongside client-certificate.
• Changed inputValidator.ts from process.env['AZURE_CLIENT_SECRET'] to core.getInput('client-secret').
• Updated tests to use setupInputMocks({ 'client-secret': ... }) instead of setting the env var.
• Updated workflow samples in sample/purview-scan.yml, README.md, sample/README.md, and Instructions.md to document and demonstrate the new input.

Cap workflow run pagination:

• Added maxRuns = 20 limit to findLastProcessedCommitSha(). The loop now stops after checking 20 runs (2 pages of 10), which is sufficient since the query is already
  branch-scoped.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass (180/180), and existing tests were updated where needed
• The dist/ folder has been rebuilt (npm run build && npm run package)
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.