ref: Add internal sensitive-data denylist and key-value filter utility

[jamescrosswell]

Closes #5421 (part of #5420).

First step of the DataCollection migration: adds the internal sensitive-terms denylist and the shared key-value filtering utility that later PRs will wire into header, cookie and query-string collection (#5422, #5423, #5424).

• SensitiveDataFilter — canonical sensitive-key denylist from the spec (case-insensitive substring matching), cookie-name-only extra terms, GDPR/PII terms (for the future SendDefaultPii=false bridge, DataCollection phase 1: resolver and SendDefaultPii bridge #5426), and FilterKeyValueData/FilterValue implementing the spec's filtering semantics: key names always preserved, sensitive values replaced with [Filtered].
• KeyValueFilterBehavior — internal off / denyList / allowList behavior (precursor to the public behavior type that ships with the DataCollection option, DataCollection phase 1: add option types (no-op) #5425). The built-in denylist always applies, including in allowList mode.

Internal only — no public API change, and nothing is wired up yet, so no behavior change either. Mirrors filterKeyValueData.ts / filtering-snippets.ts from the JS reference implementation (getsentry/sentry-javascript#20965, getsentry/sentry-javascript#20989) so the two SDKs filter identically.

#skip-changelog

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[codecov]

Codecov Report

❌ Patch coverage is 97.53086% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.69%. Comparing base (65c0c59) to head (c5002e8).

Files with missing lines	Patch %	Lines
...try/Internal/DataCollection/SensitiveDataFilter.cs	97.22%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5440      +/-   ##
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+ Coverage   74.58%   74.69%   +0.10%     
==========================================
  Files         512      514       +2     
  Lines       18666    18747      +81     
  Branches     3659     3668       +9     
==========================================
+ Hits        13922    14003      +81     
  Misses       3869     3869              
  Partials      875      875              

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