…L to fix invalid-URL error in 2.23.0 cp-7.78.0 (#30463)

## Summary

`@segment/analytics-react-native` 2.23.0 introduced a strict
`validateURL` regex via [PR
#1157](segmentio/analytics-react-native#1157)
that only allows `[a-zA-Z0-9_.-]` in query-param values. The MetaMask
Segment proxy URL encodes the write key as standard base64 in a query
param (`?b=<base64>==`), and the trailing `=` padding characters are
rejected by this regex.

When the URL fails validation `SegmentDestination.getEndpoint()`
silently falls back to `https://api.segment.io/v1/b`. Events reach
Segment's default endpoint but the proxy write key is only valid through
`fn.segmentapis.com`, so they are rejected — causing **no events to
appear in Mixpanel**.

## Change

Added `normalizeProxyUrl` in `platform-adapter.ts` that strips trailing
`=` padding from query-param values before passing the URL to the
Segment client config.

- Stripping base64 padding is safe: decoders infer it from data length
and the proxy server accepts both forms.
- The `=` key–value separator is preserved (the regex uses a lookahead
`(?=&|$)` to match only padding at the end of a param value).
- Contains a `TODO` to remove once upstream fixes the regex to accept
all RFC 3986 query characters.

## Test plan

- [ ] Run the app in dev mode and verify analytics events appear in
Mixpanel
- [ ] Verify no "Invalid URL has been passed" errors in the console
- [ ] Run unit tests: `yarn jest platform-adapter`


Made with [Cursor](https://cursor.com)

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> [!NOTE]
> **Medium Risk**
> Moderate risk because it changes how the Segment client is configured
and could affect where analytics events are sent if the proxy URL is
altered incorrectly, though the change is narrowly scoped and covered by
unit tests.
> 
> **Overview**
> Fixes Segment proxy URL validation failures by normalizing
`SEGMENT_PROXY_URL` before passing it to `createClient`, stripping
trailing base64 `=` padding from query-param values (while preserving
key/value separators).
> 
> Adds focused unit coverage for `normalizeProxyUrl` across common URL
shapes (single/double padding, multi-param URLs) and a wiring test to
ensure `createPlatformAdapter` passes the normalized `proxy` value into
Segment client configuration.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
ee00a41. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
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Co-authored-by: João Santos <joaosantos15@users.noreply.github.com>