fix: use JWT-shaped bearer tokens in A2A request context tests (#7331)

* fix: use JWT-shaped bearer tokens in A2A request context tests

Replace the scrubbed ***REMOVED*** placeholders with a JWT-shaped constant.
_is_jwt_token() drops opaque tokens, so three tests received None.
Build the token at runtime from three meaningless base64url segments.

Signed-off-by: Jonathan Springer <jps@s390x.com>

* refactor: build JWT-shaped test token with a base64url helper

Encode the header, payload and sig placeholder words at runtime.
Shorten the explanatory comment.

Signed-off-by: Jonathan Springer <jps@s390x.com>

* chore: update Rust dependencies and supply-chain audits for CI

Signed-off-by: Jonathan Springer <jps@s390x.com>

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Signed-off-by: Jonathan Springer <jps@s390x.com>

Author: jonpspri