Outbound HTTP sessions ignore proxy environment variables

[JGoutin]

Found while investigating #135, present in v1.15.0.

Three aiohttp.ClientSession constructions omit trust_env=True:

• stdapi/aws_bedrock_mantle.py — mantle_http_session()
• stdapi/region_routing.py:357
• stdapi/aws.py:580

aiohttp reads HTTPS_PROXY/HTTP_PROXY/NO_PROXY only when trust_env=True,
whereas botocore honours them unconditionally. So on a network where egress is only
possible through a proxy, the botocore paths work and every aiohttp path fails —
which is precisely the shape of #135's report: classic Bedrock reachable, Bedrock
Mantle unreachable in every region at once.

Measured with HTTPS_PROXY=http://127.0.0.1:9 (a dead port):

• Mantle succeeded in 0.47 s — it ignored the proxy entirely
• botocore failed with ProxyConnectionError in 9.78 s

Both halves are wrong: in a proxied network the first becomes a failure, and today
the two stacks simply disagree about the deployment's own network policy.

trust_env=True also enables .netrc lookups, so that is a deliberate decision to
state rather than a flag to set silently.

Needs a documentation line naming bedrock-mantle.<region>.api.aws and
com.amazonaws.<region>.bedrock-mantle as egress requirements, plus a
troubleshooting entry — an operator behind a proxy meets this as "Mantle models are
missing".