fix(bdd): resolve server address and credentials only from env

[chengxilo]

Which issue does this PR address?

Closes #3883

Rationale

A fallback turns a configuration bug into either a misleading connection error
or, worse, a green run against the wrong server. Tests should be loud when they
are broken.

What changed?

Each suite now reads those values through a required-env helper that aborts when
a variable is unset or empty, and the image no longer carries defaults. The PHP
path moves onto the key names the other seven SDKs already use
(IGGY_TCP_ADDRESS, IGGY_ROOT_USERNAME, IGGY_ROOT_PASSWORD); the PHP SDK
suite keeps its own defaults, so only the names change there. Two CI steps run
bdd/go outside run-bdd-tests.sh and were relying on the removed defaults,
one of them with no environment block at all; both now pass the variables
explicitly.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Claude (Claude Code).
2. Implementation
3. I reviewed the code line by line. There was some changes to PHP unit test by AI but I reverted them. I am not planning to make the unit test be affected by this change since they don't have ./scripts/run-bdd-tests.sh to handle the chores. It's kinda annoying to export some environment variable everytime you want to run the unit tests.
4. Yes

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

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See CONTRIBUTING.md for details.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.79%. Comparing base (dde679a) to head (b3a78b6).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3935      +/-   ##
============================================
- Coverage     84.95%   84.79%   -0.16%     
  Complexity     1405     1405              
============================================
  Files          1225     1225              
  Lines        179727   179727              
  Branches     146015   146016       +1     
============================================
- Hits         152685   152401     -284     
- Misses        23017    23273     +256     
- Partials       4025     4053      +28     

Components	Coverage Δ
Rust Core	85.79% <ø> (-0.06%)	⬇️
Java SDK	67.35% <ø> (ø)
C# SDK	75.32% <ø> (-0.08%)	⬇️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	94.48% <ø> (-1.68%)	⬇️
Go SDK	69.36% <ø> (+0.03%)	⬆️
see 42 files with indirect coverage changes

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