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 83.66%. Comparing base (c0c7493) to head (2906e83).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3935      +/-   ##
============================================
- Coverage     83.87%   83.66%   -0.21%     
  Complexity     1358     1358              
============================================
  Files          1212     1212              
  Lines        166843   166845       +2     
  Branches     134306   134432     +126     
============================================
- Hits         139937   139598     -339     
- Misses        23266    23480     +214     
- Partials       3640     3767     +127     

Components	Coverage Δ
Rust Core	84.65% <ø> (ø)
Java SDK	66.67% <ø> (ø)
C# SDK	75.06% <ø> (-1.49%)	⬇️
Python SDK	90.13% <ø> (ø)
PHP SDK	84.48% <ø> (ø)
Node SDK	93.93% <ø> (-1.92%)	⬇️
Go SDK	68.32% <ø> (ø)
see 41 files with indirect coverage changes

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