SYN-6770: preserve request redaction

[borisvalo]

Resolves #SYN-6770

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Before the change?

• Sanitized v2 request details could still expose API-check header-map values such as X-SF-TOKEN.
• Browser update payloads could not send explicit certificateIds:[] to clear attached client certificates.

After the change?

• All values under JSON headers maps are redacted in sanitized request details.
• Existing content, password, and value request-body redaction is preserved.
• Browser update payloads can distinguish omitted certificateIds from explicit certificateIds:[] without changing the public CertificateIDs []int field type.

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

PASS
coverage: 72.8% of statements
ok  	github.com/splunk/syntheticsclient/v2/syntheticsclientv2	1.431s	coverage: 72.8% of statements

Does this introduce a breaking change?

• Yes
• No

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