bug: honor PORTKEY_BASE_URL env var in set_base_url

[anxkhn]

Title: Honor the PORTKEY_BASE_URL environment variable in set_base_url

Description:

• set_base_url (portkey_ai/api_resources/utils.py) passed the PORTKEY_BASE_URL
  URL value constant ("https://api.portkey.ai/v1") to os.environ.get instead of
  the env-var name. It therefore looked up an environment variable literally named
  https://api.portkey.ai/v1 (always None), so the PORTKEY_BASE_URL environment
  variable was silently never read.
• Add a PORTKEY_BASE_URL_ENV = "PORTKEY_BASE_URL" name constant in
  global_constants.py, next to the existing PORTKEY_API_KEY_ENV and
  PORTKEY_PROXY_ENV, and use it in set_base_url. This matches the codebase's
  existing _ENV-suffixed env-var-name convention.
• Precedence is unchanged: explicit base_url argument > PORTKEY_BASE_URL env >
  PORTKEY_PROXY env > api_key-gated default (PORTKEY_BASE_URL value when an
  api_key is present, otherwise LOCAL_BASE_URL).
• Add offline regression tests (tests/test_set_base_url.py) covering env
  resolution, precedence, and the defaults.

Motivation:
Before this fix, setting PORTKEY_BASE_URL (the natural env var, and the one the
SDK's own tests and examples/demo.py use) to point at a self-hosted gateway had no
effect: the client fell through to http://localhost:8787/v1 (no api_key) or the
public cloud URL https://api.portkey.ai/v1 (with an api_key), so requests silently
went to the wrong host. Only PORTKEY_PROXY worked as an env override. Reading the
correctly-named variable makes PORTKEY_BASE_URL behave as documented and as the
tests/examples already assume.

The other two references to the PORTKEY_BASE_URL value constant are intentionally
left untouched, because they legitimately use the URL value (not the env-var name):
default_api_key's base_url == PORTKEY_BASE_URL comparison, and
default_base_url's os.environ.get(PORTKEY_PROXY_ENV, PORTKEY_BASE_URL) default.

Related Issues:
None found (no existing open issue). Confirmed at runtime against main.

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Test plan (in the PR / for reviewers)

python -m venv .venv && source .venv/bin/activate
pip install -e ".[dev]"
python -m pytest tests/test_set_base_url.py -q   # 7 passed
make lint                                        # mypy + black --check + ruff, all clean

Red -> green: reverting only the two source files (keeping the new test) makes the 3
env-honoring assertions fail with the exact buggy output (localhost / public-cloud /
PROXY fallthrough); the 4 guard tests pass on both the pre-fix and fixed code, so
they are not tautological.

[Copilot]

Pull request overview

Fixes set_base_url so it correctly reads the PORTKEY_BASE_URL environment variable (by using an env-var-name constant rather than the base URL value), aligning behavior with the documented/env-based configuration flow and preventing silent fall-through to unintended hosts.

Changes:

• Add PORTKEY_BASE_URL_ENV = "PORTKEY_BASE_URL" to centralize the env-var name alongside existing *_ENV constants.
• Update set_base_url to read os.environ.get(PORTKEY_BASE_URL_ENV) (and then fall back to PORTKEY_PROXY_ENV).
• Add regression tests validating env resolution, precedence, and default behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/test_set_base_url.py	Adds regression tests for set_base_url env handling and precedence.
portkey_ai/api_resources/utils.py	Fixes env lookup to use the env-var name constant (PORTKEY_BASE_URL_ENV) rather than the URL value.
portkey_ai/api_resources/global_constants.py	Introduces PORTKEY_BASE_URL_ENV constant consistent with existing env-var-name constants.

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